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Filed Pursuant to Rule 497
Securities Act File No. 333-218040

              PROSPECTUS SUPPLEMENT
(to Prospectus dated July 13, 2018)

$100,000,000

New Mountain Finance Corporation

5.75% Convertible Notes due 2023

New Mountain Finance Corporation ("NMFC", the "Company", "we", "us" and "our") is a Delaware corporation that was originally incorporated on June 29, 2010. We are a closed-end, non-diversified management investment company that has elected to be regulated as a business development company under the Investment Company Act of 1940, as amended (the "1940 Act"). Our investment objective is to generate current income and capital appreciation through the sourcing and origination of debt securities at all levels of the capital structure, including first and second lien debt, notes, bonds and mezzanine securities. Our first lien debt may include traditional first lien senior secured loans or unitranche loans. Unitranche loans combine characteristics of traditional first lien senior secured loans as well as second lien and subordinated loans. Unitranche loans will expose us to the risks associated with second lien and subordinated loans to the extent we invest in the "last out" tranche. In some cases, our investments may also include equity interests. Our primary focus is in the debt of defensive growth companies, which are defined as generally exhibiting the following characteristics: (i) sustainable secular growth drivers, (ii) high barriers to competitive entry, (iii) high free cash flow after capital expenditure and working capital needs, (iv) high returns on assets and (v) niche market dominance.

The investments that we invest in are almost entirely rated below investment grade or may be unrated, which are often referred to as "leveraged loans", "high yield" or "junk" debt investments, and may be considered "high risk" or speculative compared to debt investments that are rated investment grade. Such issuers are considered more likely than investment grade issuers to default on their payments of interest and principal and such risk of default could reduce our net asset value and income distributions. Our investments are also primarily floating rate debt investments that contain interest reset provisions that may make it more difficult for borrowers to make debt repayments to us if interest rates rise. In addition, some of our debt investments will not fully amortize during their lifetime, which could result in a loss or a substantial amount of unpaid principal and interest due upon maturity. Our debt investments may also lose significant market value before a default occurs. Furthermore, an active trading market may not exist for these securities. This illiquidity may make it more difficult to value our investments.

We are offering $100.0 million aggregate principal amount of our 5.75% Convertible Notes due 2023 (the "Convertible Notes"). The Convertible Notes will bear interest at a rate of 5.75% per year, payable on February 15 and August 15 of each year, commencing on February 15, 2019. The Convertible Notes will mature on August 15, 2023.

The Convertible Notes will be convertible, at your option, into shares of our common stock initially at a conversion rate of 65.8762 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $15.18 per share), subject to adjustment as described in this prospectus supplement, at any time on or prior to the close of business on the business day immediately preceding the maturity date. In the case of Convertible Notes that are converted in connection with certain types of fundamental changes, we will, in certain circumstances, increase the conversion rate by a number of additional shares.

We may not redeem the Convertible Notes prior to May 15, 2023. On or after May 15, 2023, we may redeem the notes for cash, in whole or from time to time in part, at our option at a redemption price equal to the sum of (i) 100.0% of the principal amount of the Convertible Notes to be redeemed, (ii) accrued and unpaid interest thereon to, but excluding, the redemption date and (iii) the make-whole premium (as defined herein). No sinking fund will be provided for the Convertible Notes.

You may require us to repurchase all or a portion of your Convertible Notes upon a fundamental change at a cash repurchase price equal to 100.0% of the principal amount plus accrued and unpaid interest (including additional interest, if any) through, and including, the maturity date. See "Description of the Notes — Fundamental Change Put".

The Convertible Notes will be our unsecured obligations. As of August 13, 2018, we had $1,095.3 million of indebtedness outstanding, $655.0 million of which was secured indebtedness and $440.3 million of which was unsecured indebtedness. The Convertible Notes will be our direct unsecured obligations and rank pari passu, or equally in right of payment, with all outstanding and future unsecured, unsubordinated indebtedness issued by us.

There is no public market for the Convertible Notes, and we do not intend to apply for listing of the Convertible Notes on any securities exchange or for inclusion of the Convertible Notes in any automated quotation system. Our common stock is listed on the New York Stock Exchange under the symbol "NMFC". On August 13, 2018 the last reported sales price on the New York Stock Exchange for our common stock was $14.05 per share, and the net asset value per share of our common stock on June 30, 2018 (the last date prior to the date of this prospectus supplement on which we determined our net asset value per share) was $13.57.

The price at issuance of the Convertible Notes will be 100.0% of the principal amount, plus accrued and unpaid interest, if any, from August 20, 2018.

An investment in the Convertible Notes involves risks that are described in the "Supplementary Risk Factors" section beginning on page S-25 in this prospectus supplement and the "Risk Factors" section beginning on page 27 of the accompanying prospectus.

This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in the Convertible Notes. Please read this prospectus supplement and the accompanying prospectus before investing and keep it for future reference. We file annual, quarterly and current reports, proxy statements and other information with the United States Securities and Exchange Commission (http://www.sec.gov), which is available free of charge by contacting us by mail at 787 Seventh Avenue, 48th Floor, New York, New York 10019 or on our website at http://www.newmountainfinance.com. Information contained on our website is not incorporated by reference into this prospectus supplement and the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement and the accompanying prospectus.

Neither the United States Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

We have granted the underwriters an option to purchase up to an additional $15.0 million aggregate principal amount of Convertible Notes on the same terms and conditions as set forth above, exercisable within 30 days from the date of this prospectus supplement. If the underwriters exercise this option in full, the total public offering price will be $115,000,000, the total sales load (discounts and commissions) paid by us will be $575,000, and total proceeds, before expenses, will be $114,425,000.

THE CONVERTIBLE NOTES ARE NOT DEPOSITS OR OTHER OBLIGATIONS OF A BANK AND ARE NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENT AGENCY.

Delivery of the Convertible Notes in book-entry form only through The Depository Trust Company will be made on or about August 20, 2018.

Sole Bookrunner

Wells Fargo Securities

Prospectus Supplement dated August 15, 2018

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ABOUT THIS PROSPECTUS SUPPLEMENT

          You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to buy, these securities by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of these securities. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

          This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering and also adds to and updates information contained in the accompanying prospectus. The second part is the accompanying prospectus, which gives more general information and disclosure. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control. Please carefully read this prospectus supplement and the accompanying prospectus together with any exhibits and the additional information described under "Available Information", "Prospectus Supplement Summary" and "Supplementary Risk Factors" in this prospectus supplement and the "Available Information", "Summary" and "Risk Factors" sections of the accompanying prospectus before you make an investment decision. Unless otherwise indicated, all information included in this prospectus supplement assumes no exercise by the underwriters of their option to purchase up to an additional $15,000,000 aggregate principal amount of Convertible Notes.

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SUPPLEMENTARY RISK FACTORS

          Investing in the Convertible Notes involves a number of significant risks. In addition to the other information contained in this prospectus supplement and the accompanying prospectus, you should consider carefully the following information before making an investment in the Convertible Notes. The risks set out below are not the only risks we face and you should read the risks set out in the accompanying prospectus. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the following events occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline and you may lose all or part of your investment.

RISKS RELATING TO THE NOTES

Our amount of debt outstanding will increase as a result of this offering, which could adversely affect our business, financial condition and results of operations and our ability to meet our payment obligations under the Convertible Notes and our other debt.

          As of August 13, 2018, we had $1,095.3 million of indebtedness outstanding, $655.0 million of which was secured indebtedness and $440.3 million of which was unsecured indebtedness. The use of debt could have significant consequences on our future operations, including:

•

making it more difficult for us to meet our payment and other obligations under the Convertible Notes and our other outstanding debt;



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resulting in an event of default if we fail to comply with the financial and other restrictive covenants contained in our debt agreements, which event of default could result in all of our debt becoming immediately due and payable;



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reducing the availability of our cash flow to fund investments, acquisitions and other general corporate purposes, and limiting our ability to obtain additional financing for these purposes;



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subjecting us to the risk of increased sensitivity to interest rate increases on our indebtedness with variable interest rates, including borrowings under our credit facilities; and



•

limiting our flexibility in planning for, or reacting to, and increasing our vulnerability to, changes in our business, the industry in which we operate and the general economy.

          Any of the above-listed factors could have an adverse effect on our business, financial condition and results of operations and our ability to meet our payment obligations under the Convertible Notes and our other debt. Our ability to meet our payment and other obligations under our debt instruments depends on our ability to generate significant cash flow in the future. This, to some extent, is subject to general economic, financial, competitive, legislative and regulatory factors as well as other factors that are beyond our control. We cannot assure you that our business will generate cash flow from operations, or that future borrowings will be available to us under our credit facilities or otherwise, in an amount sufficient to enable us to meet our payment obligations under the Convertible Notes and our other debt and to fund other liquidity needs. If we are not able to generate sufficient cash flow to service our debt obligations, we may need to refinance or restructure our debt, including the Convertible Notes, sell assets, reduce or delay capital investments, or seek to raise additional capital. If we are unable to implement one or more of these alternatives, we may not be able to meet our payment obligations under the Convertible Notes and our other debt.

If we default on our obligations to pay our other indebtedness, we may not be able to make payments on the Convertible Notes.

          Any default under the agreements governing our indebtedness or other indebtedness to which we may be a party that is not waived by the required lenders or holders, and the remedies sought by the

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holders of such indebtedness could make us unable to pay principal, premium, if any, and interest on the Convertible Notes and substantially decrease the market value of the Convertible Notes. If we are unable to generate sufficient cash flow and are otherwise unable to obtain funds necessary to meet required payments of principal, premium, if any, and interest on our indebtedness, or if we otherwise fail to comply with the various covenants, including financial and operating covenants, in the instruments governing our indebtedness, we could be in default under the terms of the agreements governing such indebtedness. In the event of such default, the holders of such indebtedness could elect to declare all the funds borrowed thereunder to be due and payable, together with accrued and unpaid interest, the lenders under our secured credit facilities could elect to terminate their commitments, cease making further loans, and institute foreclosure proceedings against our assets, and we could be forced into bankruptcy or liquidation. If our operating performance declines, we may in the future need to seek to obtain waivers from the required holders of our debt to avoid being in default. If we breach our covenants under our debt and seek a waiver, we may not be able to obtain a waiver from the required lenders or holders. If this occurs, we would be in default and our lenders or debt holders could exercise their rights as described above, and we could be forced into bankruptcy or liquidation. If we are unable to repay debt, lenders having secured obligations could proceed against the collateral securing the debt. Because our credit facilities, 2014 Convertible Notes, and Unsecured Notes have, and any future credit facilities will likely have, customary cross-default provisions, if the indebtedness thereunder or under any future credit facility is accelerated, we may be unable to repay or finance the amounts due.

The Convertible Notes will be unsecured and therefore will be effectively subordinated to any secured indebtedness we have currently incurred or may incur in the future.

          The Convertible Notes will not be secured by any of our assets or any of the assets of our subsidiaries. As a result, the Convertible Notes are effectively subordinated to any secured indebtedness we or our subsidiaries have outstanding as of the date of this prospectus supplement or that we or they may incur in the future (or any indebtedness that is initially unsecured to which we subsequently grant security) to the extent of the value of the assets securing such indebtedness. In any liquidation, dissolution, bankruptcy or other similar proceeding, the holders of any of our existing or future secured indebtedness and the secured indebtedness of our subsidiaries may assert rights against the assets pledged to secure that indebtedness in order to receive full payment of their indebtedness before the assets may be used to pay other creditors, including the holders of the Convertible Notes. As of August 13, 2018, we had $1,095.3 million of indebtedness outstanding, $655.0 million of which was secured indebtedness, and therefore effectively senior to the Convertible Notes to the extent of the value of such assets, and $440.3 million of which was unsecured indebtedness.

The Convertible Notes will be structurally subordinated to the indebtedness and other liabilities of our subsidiaries.

          The Convertible Notes are obligations exclusively of NMFC and not of any of our subsidiaries. None of our subsidiaries is a guarantor of the Convertible Notes and the Convertible Notes are not required to be guaranteed by any subsidiaries we may acquire or create in the future. Except to the extent we are a creditor with recognized claims against our subsidiaries, all claims of creditors, including trade creditors, and holders of preferred stock, if any, of our subsidiaries will have priority over our equity interests in such subsidiaries (and therefore the claims of our creditors, including holders of the Convertible Notes) with respect to the assets of such subsidiaries. Even if we were recognized as a creditor of one or more of our subsidiaries, our claims would still be effectively subordinated to any security interests in the assets of any such subsidiary and to any indebtedness or other liabilities of any such subsidiary senior to our claims. Consequently, the Convertible Notes will be subordinated structurally to all indebtedness and other liabilities, including trade payables, of any of our subsidiaries and any subsidiaries that we may in the future acquire or establish as financing vehicles or otherwise. All of the existing indebtedness of our subsidiaries would be structurally senior to the Convertible Notes. In

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addition, our subsidiaries may incur substantial additional indebtedness in the future, all of which would be structurally senior to the Convertible Notes.

The indenture governing the Convertible Notes contains limited restrictive covenants and provides only limited protection in the event of a change of control.

          The indenture under which the Convertible Notes will be issued contains limited financial covenants and does not contain operating covenants or any other restrictive covenants that would limit our or our subsidiaries' ability to engage in certain transactions that may adversely affect your investment in the Convertible Notes. In particular, without limitation, the indenture does not place any restrictions on our or any of our subsidiaries' ability to:

•

unless, after giving effect to such incurrence, we will have a Debt to Equity Ratio of not greater than 1.65 to 1.00 (as set forth under "Description of the Notes — Certain Covenants — Debt to Equity Ratio" in this prospectus supplement), incur additional indebtedness or other obligations, including (1) any indebtedness or other obligations that would be equal in right of payment to the Convertible Notes, (2) any indebtedness or other obligations that would be secured and therefore rank effectively senior in right of payment to the Convertible Notes to the extent of the values of the assets securing such debt, (3) indebtedness of ours that is guaranteed by one or more of our subsidiaries and which therefore is structurally senior to the Convertible Notes and (4) securities, indebtedness or obligations issued or incurred by our subsidiaries that would be senior to our equity interests in our subsidiaries and therefore rank structurally senior to the Convertible Notes with respect to the assets of our subsidiaries;



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pay dividends on, or purchase or redeem or make any payments in respect of, capital stock or other securities ranking junior in right of payment to the Convertible Notes, including subordinated indebtedness;



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sell assets (other than certain limited restrictions on our ability to consolidate, merge or sell all or substantially all of our assets);



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enter into transactions with affiliates;



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create liens (including liens on the shares of our subsidiaries) or enter into sale and leaseback transactions;



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make investments; or



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create restrictions on the payment of dividends or other amounts to us from our subsidiaries.

          However, we must maintain a Secured Debt Ratio of not greater than 0.70 to 1.00 at all times. See "Description of the Notes — Certain Covenants — Maximum Secured Debt" In this prospectus supplement.

          We will only be required to offer to repurchase the Convertible Notes upon a change of control in the case of the transactions specified in the definition of a "fundamental change" under "Description of the Notes — Fundamental Change Put". Similarly, we will only be required to adjust the conversion rate upon the occurrence of a "non-stock change of control" in circumstances where a Convertible Note is converted in connection with such a transaction as set forth under "Description of the Notes — Conversion Rights — Adjustment to Conversion Rate Upon a Non-Stock Change of Control". Accordingly, subject to restrictions contained in our other debt agreements, we will be permitted to engage in certain transactions, such as acquisitions, re-financings or recapitalizations, that could affect our capital structure and the value of the Convertible Notes and our common stock but would not constitute a fundamental change under the Convertible Notes.

          Furthermore, the terms of the indenture and the Convertible Notes do not protect holders of the Convertible Notes in the event that we experience changes (including significant adverse changes) in

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our financial condition, results of operations or credit ratings, as they do not require that we or our subsidiaries adhere to any financial tests or ratios or specified levels of net worth, revenues, income, cash flow or liquidity. Our ability to recapitalize, incur additional debt and take a number of other actions that are not limited by the terms of the Convertible Notes may have important consequences for you as a holder of the Convertible Notes, including making it more difficult for us to satisfy our obligations with respect to the Convertible Notes or negatively affecting the trading value of the Convertible Notes. Certain of our current debt instruments include more protections for their holders than the indenture and the Convertible Notes. In addition, other debt we issue or incur in the future could contain more protections for its holders than the indenture and the Convertible Notes, including additional covenants and events of default. The issuance or incurrence of any such debt with incremental protections could affect the market for and trading levels and prices of the Convertible Notes.

The conversion rate of the Convertible Notes may not be adjusted for all dilutive events that may adversely affect the trading price of the Convertible Notes or the common stock issuable upon conversion of the Convertible Notes.

          The conversion rate of the Convertible Notes is subject to adjustment upon certain events, including the issuance of certain stock dividends on our common stock, certain issuance of rights or warrants, subdivisions, combinations, certain distributions of capital stock, indebtedness or assets, certain cash dividends and certain issuer tender or exchange offers as described under "Description of the Notes — Conversion Rights — Conversion Rate Adjustments". The conversion rate will not be adjusted for certain other events, including cash dividends below the dividend threshold amount (as defined in clause (4) of "Description of the Notes — Conversion Rights — Conversion Rate Adjustments"), which may adversely affect the trading price of the Convertible Notes or the common stock issuable upon conversion of the Convertible Notes.

We may be unable to repurchase the Convertible Notes following a fundamental change.

          Holders of the Convertible Notes have the right to require us to repurchase the Convertible Notes prior to their maturity upon the occurrence of a fundamental change as described under "Description of the Notes — Fundamental Change Put". Any of our future debt agreements may contain similar provisions. We may not have sufficient funds or the ability to arrange necessary financing on acceptable terms at the time we are required to make repurchases of tendered Convertible Notes. In addition, our ability to repurchase the Convertible Notes may be limited by law or the terms of other agreements relating to our debt outstanding at the time, including our credit facilities. Under certain of our existing credit facilities, we would be prohibited from making any such repurchase without consent from the lenders thereunder or a waiver or modification of such requirements. If we fail to repurchase the Convertible Notes as required by the indenture, it would constitute an event of default under the indenture governing the Convertible Notes.

Some significant restructuring transactions may not constitute a fundamental change, in which case we would not be obligated to offer to repurchase the Convertible Notes.

          Upon the occurrence of a fundamental change, you have the right to require us to offer to repurchase the Convertible Notes. However, the fundamental change provisions will not afford protection to holders of the Convertible Notes in the event of certain transactions. For example, transactions such as leveraged recapitalizations, re-financings, restructurings or acquisitions initiated by us would not constitute a fundamental change requiring us to repurchase the Convertible Notes. In the event of any such transaction, the holders would not have the right to require us to repurchase the Convertible Notes, even though each of these transactions could increase the amount of our indebtedness, or otherwise adversely affect our capital structure or any credit ratings, thereby adversely affecting the holders of the Convertible Notes.

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Provisions of the Convertible Notes could discourage an acquisition of us by a third party.

          Certain provisions of the Convertible Notes could make it more difficult or more expensive for a third party to acquire us. Upon the occurrence of certain transactions constituting a fundamental change, holders of the Convertible Notes will have the right, at their option, to require us to repurchase all of their Convertible Notes or any portion of the principal amount of such Convertible Notes in integral multiples of $1,000. We may also be required to increase the conversion rate or provide for conversion into the acquirer's capital stock in the event of certain fundamental changes. These provisions could discourage an acquisition of us by a third party.

The adjustment to the conversion rate upon the occurrence of certain types of fundamental changes may not adequately compensate you for the lost option time value of your Convertible Notes as a result of such fundamental change.

          If certain types of fundamental changes occur on or prior to the maturity date of the Convertible Notes, we may increase the conversion rate by an additional number of shares for holders that elect to convert their Convertible Notes in connection with the fundamental change. The number of additional shares to be added to the conversion rate will be determined based on the date on which the fundamental change becomes effective and the price paid per share of our common stock in the fundamental change as described under "Description of the Notes — Conversion Rights — Adjustment to Conversion Rate Upon a Non-Stock Change of Control". Although this adjustment is designed to compensate you for the lost option value of your Convertible Notes as a result of certain types of fundamental changes, the adjustment is only an approximation of such lost value based upon assumptions made on the date of this prospectus supplement and may not adequately compensate you for such loss. In addition, if the price paid per share of our common stock in the fundamental change is less than $13.80 or more than $18.00 (subject to adjustment), there will be no such adjustment.

There is currently no public market for the Convertible Notes, and an active trading market may not develop. The failure of a market to develop for the Convertible Notes could adversely affect the liquidity and value of your Convertible Notes.

          There is currently no public market for the Convertible Notes, and we do not intend to apply for listing of the Convertible Notes on any securities exchange or for quotation of the Convertible Notes on any automated dealer quotation system. We have been advised by the underwriters that, following the completion of the offering, they currently intend to make a market in the Convertible Notes. However, the underwriters are not obligated to do so and any market-making activities with respect to the Convertible Notes may be discontinued at any time without notice. In addition, any market-making activity will be subject to limits imposed by law. A market may not develop for the Convertible Notes, and there can be no assurance as to the liquidity of any market that may develop for the Convertible Notes. If an active, liquid market does not develop for the Convertible Notes, the market price and liquidity of the Convertible Notes may be adversely affected. If any of the Convertible Notes are traded after their initial issuance, they may trade at a discount from their initial offering price.

          The liquidity of the trading market, if any, and future trading prices of the Convertible Notes will depend on many factors, including, among other things, the market price of our common stock, prevailing interest rates, our operating results, financial performance and prospects, the market for similar securities and the overall securities market, and may be adversely affected by unfavorable changes in these factors. Historically, the market for convertible debt has been subject to disruptions that have caused volatility in prices. It is possible that the market for the Convertible Notes will be subject to disruptions which may have a negative effect on the holders of the Convertible Notes, regardless of our operating results, financial performance or prospects.

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Regulatory actions may adversely affect the trading price and liquidity of the Convertible Notes.

          We expect that many investors in the Convertible Notes will employ, or seek to employ, a convertible arbitrage strategy with respect to the Convertible Notes. Investors that employ a convertible arbitrage strategy with respect to convertible debt instruments typically implement that strategy by selling short the common stock underlying the convertible notes and dynamically adjusting their short position while they hold the notes. Investors may also implement this strategy by entering into swaps on the common stock in lieu of or in addition to short selling the common stock. As a result, any specific rules regulating short selling of securities or equity swaps or other governmental action that interfere with the ability of market participants to effect short sales or equity swaps with respect to our common stock could adversely affect the ability of investors in the Convertible Notes to conduct the convertible arbitrage strategy that we believe they will employ, or seek to employ, with respect to the Convertible Notes. This could, in turn, adversely affect the trading price and liquidity of the Convertible Notes.

          The SEC and other regulatory and self-regulatory authorities have implemented various rule changes and may adopt additional rule changes in the future that may impact those engaging in short-selling activity involving equity securities (including our common stock), including Rule 201 of SEC Regulation SHO, the Financial Industry Regulatory Authority, Inc.'s "Limit Up-Limit Down" program, market-wide circuit breaker systems that halt trading of stock for certain periods following specific market declines, and rules stemming from the enactment and implementation of the Dodd-Frank Wall Street Reform and Consumer Protection Act. Past regulatory actions, including emergency actions or regulations, have had a significant impact on the trading prices and liquidity of equity-linked instruments. Any governmental action that similarly restricts the ability of investors in, or potential purchasers of, the Convertible Notes to effect short sales of our common stock could similarly adversely affect the trading price and the liquidity of the Convertible Notes.

The accounting for convertible debt securities is subject to uncertainty.

          The accounting for convertible debt securities is subject to frequent scrutiny by the accounting regulatory bodies and is subject to change. We cannot predict if or when any such change could be made and any such change could have an adverse impact on our reported or future financial results. Any such impacts could adversely affect the market price of our common stock and in turn negatively impact the trading price of the Convertible Notes.

The price of our common stock and of the Convertible Notes may fluctuate significantly, and this may make it difficult for you to resell the Convertible Notes or common stock issuable upon conversion of the Convertible Notes when you want or at prices you find attractive.

          The price of our common stock constantly changes and may fluctuate significantly. We expect that the market price and liquidity of our common stock will continue to fluctuate. In addition, because the Convertible Notes are convertible into our common stock, volatility or depressed prices for our common stock could have a similar effect on the trading price of the Convertible Notes. Our stock price and its liquidity may fluctuate as a result of a variety of factors, many of which are beyond our control. These factors include:

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price and volume fluctuations in the overall stock market or in the market for BDCs from time to time;



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investor demand for shares of our common stock;



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significant volatility in the market price and trading volume of securities of registered closed-end management investment companies, BDCs or other financial services companies, which is not necessarily related to the operating performance of these companies;



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our inability to borrow money or deploy or invest our capital;



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fluctuations in interest rates;



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any shortfall in revenue or net income or any increase in losses from levels expected by investors or securities analysts;



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operating performance of companies comparable to us;



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changes in regulatory policies or tax guidelines with respect to RICs or BDCs;



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our loss of status as or ability to operate as a BDC;



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our failure to qualify as a RIC, loss of RIC status or ability operate as a RIC;



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actual or anticipated changes in our earnings or fluctuations in our operating results;



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changes in the value of our portfolio of investments;



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general economic conditions, trends and other external factors;



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departures of key personnel; or



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loss of a major source of funding.

          In addition, we are required to continue to meet certain listing standards in order for our common stock to remain listed on the NYSE. If we were to be delisted by the NYSE, the liquidity of our common stock would be materially impaired.

Future sales of our common stock in the public market or the issuance of securities senior to our common stock could adversely affect the trading price of our common stock and the value of the Convertible Notes and our ability to raise funds in new stock offerings.

          Future sales of substantial amounts of our common stock or equity-related securities in the public market, or the perception that such sales could occur, could adversely affect prevailing trading prices of our common stock and the value of the Convertible Notes and could impair our ability to raise capital through future offerings of our securities, should we decide to offer them. We may not, unless otherwise agreed to by the underwriters, commence any sales of shares of our common stock until 30 days following the date of this prospectus supplement. No prediction can be made as to the effect, if any, that future sales of shares of common stock, or the availability of shares of common stock for future sale, will have on the trading price of our common stock or the value of the Convertible Notes.

Holders of the Convertible Notes will not be entitled to any rights with respect to our common stock, but will be subject to all changes made with respect to our common stock.

          Holders of the Convertible Notes will not be entitled to any rights with respect to our common stock (including, without limitation, voting rights or rights to receive any dividends or other distributions on our common stock), but will be subject to all changes affecting our common stock. Holders will only be entitled to rights in respect of our common stock if and when we deliver shares of our common stock upon conversion for their Convertible Notes and, to a limited extent, under the conversion rate adjustments applicable to the Convertible Notes. For example, in the event that an amendment is proposed to our certificate of incorporation or bylaws requiring stockholder approval and the record date for determining the stockholders of record entitled to vote on the amendment occurs prior to a holder's conversion of Convertible Notes, the holder will not be entitled to vote on the amendment, although the holder will nevertheless be subject to any changes in the powers, preferences or rights of our common stock that result from such amendment.

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You may be deemed to receive a taxable distribution without the receipt of any cash or property.

          The conversion rate of the Convertible Notes will be adjusted in certain circumstances. See the discussion under the headings "Description of the Notes — Conversion Rights — Conversion Rate Adjustments" and "— Adjustment to Conversion Rate Upon a Non-Stock Change of Control". Adjustments (or failures to make adjustments) to the conversion rate of the Convertible Notes that have the effect of increasing your proportionate interest in our assets or earnings may in some circumstances result in a taxable constructive distribution to you for U.S. federal income tax purposes, notwithstanding the fact that you do not receive an actual distribution of cash or property. In addition, if you are a Non-U.S. Holder (as defined in "Certain Material U.S. Federal Income Tax Considerations" in this prospectus supplement), you may be subject to U.S. federal withholding taxes in connection with such a constructive distribution. If we pay withholding taxes on your behalf as a result of an adjustment to the conversion rate of the Convertible Notes, we may, at our option, set off such payments against payments of cash and common stock on the Convertible Notes. You are urged to consult your tax advisors with respect to the U.S. federal income tax consequences resulting from an adjustment to the conversion rate of the Convertible Notes. See the discussions under the headings "Certain Material U.S. Federal Income Tax Considerations — Tax Consequences to U.S. Holders of Convertible Notes — Constructive distributions" and "— Tax Consequences to Non-U.S. Holders of Convertible Notes — Constructive distributions" in this prospectus supplement.

RISKS RELATED TO OUR BUSINESS AND STRUCTURE

Our ability to achieve our investment objective depends on key investment personnel of the Investment Adviser. If the Investment Adviser were to lose any of its key investment personnel, our ability to achieve our investment objective could be significantly harmed.

          We depend on the investment judgment, skill and relationships of the investment professionals of the Investment Adviser, particularly Steven B. Klinsky, Robert A. Hamwee and John R. Kline, as well as other key personnel to identify, evaluate, negotiate, structure, execute, monitor and service our investments. The Investment Adviser, as an affiliate of New Mountain Capital, is supported by New Mountain Capital's team, which as of June 30, 2018 consisted of approximately 140 employees and senior advisors of New Mountain Capital and its affiliates to fulfill its obligations to us under the Investment Management Agreement. The Investment Adviser may also depend upon New Mountain Capital to obtain access to investment opportunities originated by the professionals of New Mountain Capital and its affiliates. Our future success depends to a significant extent on the continued service and coordination of the key investment personnel of the Investment Adviser. The departure of any of these individuals could have a material adverse effect on our ability to achieve our investment objective.

          The Investment Committee, which provides oversight over our investment activities, is provided by the Investment Adviser. The Investment Committee currently consists of five members. The loss of any member of the Investment Committee or of other senior professionals of the Investment Adviser and its affiliates without suitable replacement could limit our ability to achieve our investment objective and operate as we anticipate. This could have a material adverse effect on our financial condition, results of operation and cash flows. To achieve our investment objective, the Investment Adviser may hire, train, supervise and manage new investment professionals to participate in its investment selection and monitoring process. If the Investment Adviser is unable to find investment professionals or do so in a timely manner, our business, financial condition and results of operations could be adversely affected.

We borrow money, which could magnify the potential for gain or loss on amounts invested in us and increase the risk of investing in us.

          We borrow money as part of our business plan. Borrowings, also known as leverage, magnify the potential for gain or loss on invested equity capital and may, consequently, increase the risk of investing in us. We expect to continue to use leverage to finance our investments, through senior securities issued

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by banks and other lenders. Lenders of these senior securities have fixed dollar claims on our assets that are superior to claims of our common stockholders and we would expect such lenders to seek recovery against our assets in the event of a default. If the value of our assets decreases, leveraging would cause our net asset value to decline more sharply than it otherwise would have had it not leveraged. Similarly, any decrease in our income would cause our net income to decline more sharply than it would have had it not borrowed. Such a decline could adversely affect our ability to make common stock distribution payments. In addition, because our investments may be illiquid, we may be unable to dispose of them or to do so at a favorable price in the event we need to do so if we are unable to refinance any indebtedness upon maturity and, as a result, we may suffer losses. Leverage is generally considered a speculative investment technique and increases the risks associated with investing in our securities.

          Our ability to service any debt that we incur depends largely on our financial performance and is subject to prevailing economic conditions and competitive pressures. Moreover, as the Investment Adviser's management fee is payable to the Investment Adviser based on gross assets, including those assets acquired through the use of leverage, the Investment Adviser may have a financial incentive to incur leverage which may not be consistent with our interests and the interests of our common stockholders. In addition, holders of our common stock will, indirectly, bear the burden of any increase in our expenses as a result of leverage, including any increase in the management fee payable to the Investment Adviser.

          At June 30, 2018, we had $390.5 million, $150.0 million, $155.3 million, $235.0 million, and $163.0 million of indebtedness outstanding under the Holdings Credit Facility, the NMFC Credit Facility, the 2014 Convertible Notes, the Unsecured Notes and the SBA-guaranteed debentures, respectively. The Holdings Credit Facility, the NMFC Credit Facility, the SBA-guaranteed debentures and the Unsecured Notes had weighted average interest rates of 4.0%, 4.3%, 3.2% and 5.0%, respectively, for the six months ended June 30, 2018. The interest rate on the 2014 Convertible Notes is 5.0% per annum.

          Illustration.    The following table illustrates the effect of leverage on returns from an investment in our common stock assuming various annual returns, net of expenses and adjusted for unsettled securities purchased. The calculations in the table below are hypothetical. Actual returns may be higher or lower than those appearing below. The calculation assumes (i) $2,205.9 million in total assets, (ii) a weighted average cost of borrowings of 4.3%, which assumes the weighted average interest rates as of June 30, 2018 for the Holdings Credit Facility, the NMFC Credit Facility, Unsecured Notes and the SBA-guaranteed debentures and the interest rate as of June 30, 2018 for the 2014 Convertible Notes, (iii) $1,093.8 million in debt outstanding and (iv) $1,032.6 million in net assets. This table excludes the impact of our July 5, 2018 issuance of the 2018B Unsecured Notes as the issuance occurred after June 30, 2018.

Assumed Return on Our Portfolio

(net of expenses)

If we are unable to obtain additional debt financing, or if our borrowing capacity is materially reduced, our business could be materially adversely affected.

          We may want to obtain additional debt financing, or need to do so upon maturity of our credit facilities, in order to obtain funds which may be made available for investments. The Holdings Credit Facility, the NMFC Credit Facility and the 2014 Convertible Notes mature on October 24, 2022, June 4, 2022 and June 15, 2019, respectively. Our $90.0 million in aggregate principal amount of the 2016 Unsecured Notes will mature on May 15, 2021, our $55.0 million in aggregate principal amount of the 2017A Unsecured Notes will mature on July 15, 2022, our $90.0 million in aggregate principal amount of

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the 2018A Unsecured Notes will mature on January 30, 2023 and our $50.0 million in aggregate principal amount of 2018B Unsecured Notes will mature on June 28, 2023. The SBA-guaranteed debentures have ten year maturities and will begin to mature on March 1, 2025. If we are unable to increase, renew or replace any such facilities and enter into new debt financing facilities or other debt financing on commercially reasonable terms, our liquidity may be reduced significantly. In addition, if we are unable to repay amounts outstanding under any such facilities and are declared in default or are unable to renew or refinance these facilities, we may not be able to make new investments or operate our business in the normal course. These situations may arise due to circumstances that we may be unable to control, such as lack of access to the credit markets, a severe decline in the value of the U.S. dollar, an economic downturn or an operational problem that affects us or third parties, and could materially damage our business operations, results of operations and financial condition.

RISKS RELATED TO OUR OPERATIONS

Regulations governing the operations of BDCs will affect our ability to raise additional equity capital as well as our ability to issue senior securities or borrow for investment purposes, any or all of which could have a negative effect on our investment objectives and strategies.

          Our business requires a substantial amount of capital. We may acquire additional capital from the issuance of senior securities, including borrowing under a credit facility or other indebtedness. In addition, we may also issue additional equity capital, which would in turn increase the equity capital available to us. However, we may not be able to raise additional capital in the future on favorable terms or at all.

          We may issue debt securities, preferred stock, and we may borrow money from banks or other financial institutions, which we refer to collectively as "senior securities", up to the maximum amount permitted by the 1940 Act. The 1940 Act permits us to issue senior securities in amounts such that our asset coverage, as defined in the 1940 Act, equals at least 150.0% after each issuance of senior securities. As a result of our SEC exemptive relief, we are permitted to exclude our SBA-guaranteed debentures from the definition of senior securities in the 150.0% asset coverage ratio we are required to maintain under the 1940 Act. If our asset coverage ratio is not at least 150.0% we would be unable to issue senior securities, and if we had senior securities outstanding (other than any indebtedness issued in consideration of a privately arranged loan, such as any indebtedness outstanding under the Holdings Credit Facility and NMFC Credit Facility), we would be unable to make distributions to our stockholders. However, at June 30, 2018, our only senior securities outstanding were indebtedness under the Holdings Credit Facility, NMFC Credit Facility, 2014 Convertible Notes and Unsecured Notes and therefore at June 30, 2018, we would not have been precluded from paying distributions. If the value of our assets declines, we may be unable to satisfy this test. If that happens, we may be required to liquidate a portion of our investments and repay a portion of our indebtedness at a time when such sales may be disadvantageous.

          The Holdings Credit Facility matures on October 24, 2022 and permits borrowings of $495.0 million as of June 30, 2018. The Holdings Credit Facility had $390.5 million in debt outstanding as of June 30, 2018. The NMFC Credit Facility matures on June 4, 2022 and permits borrowings of $150.0 million as of June 30, 2018. The NMFC Credit Facility had $150.0 million in debt outstanding as of June 30, 2018. The 2014 Convertible Notes mature on June 15, 2019. The 2014 Convertible Notes had $155.3 million in debt outstanding as of June 30, 2018. The 2016 Unsecured Notes, 2017A Unsecured Notes and 2018A Unsecured Notes mature on May 15, 2021, July 15, 2022 and January 30, 2023, respectively, and had $90.0 million, $55.0 million and $90.0 million, respectively, in debt outstanding as of June 30, 2018. The $50.0 million in aggregate principal amount of 2018B Unsecured Notes were issued on July 5, 2018 and mature on June 28, 2023. The SBA-guaranteed debentures have ten year maturities and will begin to mature on March 1, 2025. As of June 30, 2018, $163.0 million of SBA-guaranteed debentures were outstanding.

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          In addition, we may in the future seek to securitize other portfolio securities to generate cash for funding new investments. To securitize loans, we would likely create a wholly-owned subsidiary and contribute a pool of loans to the subsidiary. We would then sell interests in the subsidiary on a non-recourse basis to purchasers and we would retain all or a portion of the equity in the subsidiary. If we are unable to successfully securitize its loan portfolio, which must be done in compliance with the relevant restrictions in the Holdings Credit Facility, our ability to grow our business or fully execute our business strategy could be impaired and our earnings, if any, could decrease. The securitization market is subject to changing market conditions, and we may not be able to access this market when it would be otherwise deemed appropriate. Moreover, the successful securitization of our portfolio might expose us to losses as the residual investments in which we do not sell interests will tend to be those that are riskier and more apt to generate losses. The 1940 Act also may impose restrictions on the structure of any securitization.

          We may also obtain capital through the issuance of additional equity capital. As a BDC, we generally are not able to issue or sell our common stock at a price below net asset value per share. If our common stock trades at a discount to our net asset value per share, this restriction could adversely affect our ability to raise equity capital. We may, however, sell our common stock, or warrants, options or rights to acquire our common stock, at a price below our net asset value per share of the common stock if our board of directors and independent directors determine that such sale is in our best interests and the best interests of our stockholders, and our stockholders approve such sale. In any such case, the price at which our securities are to be issued and sold may not be less than a price that, in the determination of our board of directors, closely approximates the market value of such securities (less any underwriting commission or discount). If we raise additional funds by issuing more shares of our common stock, or if we issue senior securities convertible into, or exchangeable for, our common stock, the percentage ownership of our stockholders may decline and you may experience dilution.

RISKS RELATING TO OUR INVESTMENTS

Our portfolio may be concentrated in a limited number of industries, which may subject us to a risk of significant loss if there is a downturn in a particular industry in which a number of our investments are concentrated.

          Our portfolio may be concentrated in a limited number of industries. For example, as of June 30, 2018, our investments in the business services and the software industries represented approximately 32.8% and 15.7%, respectively, of the fair value of our portfolio. A downturn in any particular industry in which we are invested could significantly impact the portfolio companies operating in that industry, and accordingly, the aggregate returns that we realize from our investment in such portfolio companies.

          Specifically, companies in the business services industry are subject to general economic downturns and business cycles, and will often suffer reduced revenues and rate pressures during periods of economic uncertainty. In addition, companies in the software industry often have narrow product lines and small market shares. Because of rapid technological change, the average selling prices of products and some services provided by software companies have historically decreased over their productive lives. As a result, the average selling prices of products and services offered by software companies in which we invest may decrease over time. If an industry in which we have significant investments suffers from adverse business or economic conditions, as these industries have to varying degrees, a material portion of our investment portfolio could be affected adversely, which, in turn, could adversely affect our financial position and results of operations.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

          This prospectus supplement and the accompanying prospectus contain forward-looking statements that involve substantial risks and uncertainties. These forward-looking statements are not historical facts, but rather are based on current expectations, estimates and projections about us, our current and prospective portfolio investments, our industry, our beliefs, and our assumptions. Words such as "anticipate", "believe", "continue", "could", "estimate", "expect", "intend", "may", "plan", "potential", "project", "seek", "should", "target", "will", "would" or variations of these words and similar expressions are intended to identify forward-looking statements. The forward-looking statements contained in this prospectus supplement and the accompanying prospectus involve risks and uncertainties, including statements as to:

•

the preliminary estimates of our net asset value and adjusted net investment income;



•

our future operating results;



•

our business prospects and the prospects of our portfolio companies;



•

the impact of investments that we expect to make;



•

our contractual arrangements and relationships with third parties;



•

the dependence of our future success on the general economy and its impact on the industries in which we invest;



•

the ability of our portfolio companies to achieve their objectives;



•

our expected financings and investments;



•

the adequacy of our cash resources and working capital; and



•

the timing of cash flows, if any, from the operations of our portfolio companies.

          These statements are not guarantees of future performance and are subject to risks, uncertainties, and other factors, some of which are beyond our control and difficult to predict and could cause actual results to differ materially from those expressed or forecasted in the forward-looking statements, including without limitation:

•

an economic downturn could impair our portfolio companies' ability to continue to operate, which could lead to the loss of some or all of our investments in such portfolio companies;



•

a contraction of available credit and/or an inability to access the equity markets could impair our lending and investment activities;



•

interest rate volatility could adversely affect our results, particularly if we elect to use leverage as part of our investment strategy;



•

currency fluctuations could adversely affect the results of our investments in foreign companies, particularly to the extent that we receive payments denominated in foreign currency rather than U.S. dollars; and



•

the risks, uncertainties and other factors we identify in "Supplementary Risk Factors" in this prospectus supplement and "Risk Factors" in the accompanying prospectus, and elsewhere in this prospectus supplement, the accompanying prospectus and in our filings with the SEC.

          Although we believe that the assumptions on which these forward-looking statements are based are reasonable, any of those assumptions could prove to be inaccurate, and as a result, the forward-looking statements based on those assumptions also could be inaccurate. Important assumptions include our ability to originate new loans and investments, certain margins and levels of profitability and the availability of additional capital. In light of these and other uncertainties, the inclusion of a projection or forward-looking statement in this prospectus supplement or the accompanying prospectus should not be

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regarded as a representation by us that our plans and objectives will be achieved. These risks and uncertainties include those described or identified in "Supplementary Risk Factors" in this prospectus supplement and "Risk Factors" in the accompanying prospectus, and elsewhere in this prospectus supplement and the accompanying prospectus. You should not place undue reliance on these forward-looking statements, which apply only as of the date of this prospectus supplement. However, we will update this prospectus supplement to reflect any material changes to the information contained herein. The forward-looking statements and projections contained in this prospectus supplement are excluded from the safe harbor protection provided by Section 27A of the Securities Act.

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CAPITALIZATION

          The following table sets forth our capitalization as of June 30, 2018:

•

on an actual basis;



•

on an as adjusted basis to give effect to the issuance of $50.0 million in aggregate principal amount of 5.36% Series 2018B Notes due June 28, 2023 issued on July 5, 2018 to an institutional investor in a private placement; and



•

on an as further adjusted basis to give effect to the assumed sale of $100.0 million aggregate principal amount of Convertible Notes (assuming no exercise of the overallotment option), excluding accrued interest, after deducting the underwriting discounts and commissions of approximately $0.5 million payable by us and estimated offering expenses of approximately $0.4 million payable by us, giving effect to the use of the cash proceeds from such sale to repay outstanding indebtedness under the NMFC Credit Facility and then, to the extent any net proceeds remain, the Holdings Credit Facility, as described in "Use of Proceeds", but not reflecting any reborrowing.

          You should read this table together with "Use of Proceeds" and the financial statements and related notes thereto included elsewhere in this prospectus supplement and the accompanying prospectus.