This preliminary prospectus supplement relates to an effective registration statement under the Securities Act of 1933, as amended, but the information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell and are not soliciting an offer to buy these securities in any jurisdiction where the offer and sale is not permitted.

Exhibit 99.1

[FORM OF PRELIMINARY PROSPECTUS SUPPLEMENT TO BE USED IN

CONJUNCTION WITH FUTURE COMMON STOCK OFFERINGS]¹

PRELIMINARY PROSPECTUS SUPPLEMENT

(to Prospectus dated             , 20     )

[                    ] Shares

New Mountain Finance Corporation

Common Stock

New Mountain Finance Corporation (“NMFC”) is a holding company with no direct operations of its own, and its only business and sole asset is its ownership of common membership units of New Mountain Finance Holdings, L.L.C. (the “Operating Company”). The Operating Company is an externally managed business development company managed by New Mountain Finance Advisers BDC, L.L.C. and is the operating company for NMFC’s business. NMFC and the Operating Company each have elected to be treated as a business development company under the Investment Company Act of 1940. The Operating Company’s 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. NMFC owns approximately 34.6% of the common membership units of the Operating Company and New Mountain Finance AIV Holdings Corporation owns approximately 65.4% of the common membership units of the Operating Company.

We are offering for sale              shares of NMFC’s common stock. We have granted the underwriters a [    ]-day option to purchase up to additional shares of NMFC’s common stock at the public offering price, less the underwriting discounts and commissions.

¹ In addition to the sections outlined in this form of prospectus supplement, each prospectus supplement actually used in connection with an offering conducted pursuant to the registration statement to which this form of prospectus supplement is attached will be updated to include such other information as may then be required to be disclosed therein pursuant to applicable law or regulation as in effect as of the date of each such prospectus supplement, including, without limitation, information particular to the terms of each security offered thereby and any related risk factors or tax considerations pertaining thereto. This form of prospectus supplement is intended only to provide a rough approximation of the nature and type of disclosure that may appear in any actual prospectus supplement used for the purposes of offering securities pursuant to the registration statement to which this form of prospectus supplement is attached, and is not intended to and does not contain all of the information that would appear is any such actual prospectus supplement, and should not be used or relied upon in connection with any offer or sale of securities.

NMFC’s common stock is listed on the New York Stock Exchange under the symbol “NMFC”. On             , 20     , the last reported sales price on the New York Stock Exchange for NMFC’s common stock was $ per share.

An investment in NMFC’s common stock is very risky and highly speculative. Shares of closed-end investment companies, including business development companies, frequently trade at a discount to their net asset value. In addition, the companies in which NMFC invests, through the Operating Company, are subject to special risks. See “Risk Factors” beginning on page [ ] of the accompanying prospectus to read about factors you should consider, including the risk of leverage, before investing in NMFC’s common stock.

Neither the 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.

This prospectus supplement and the accompanying prospectus contain important information about NMFC and the Operating Company that a prospective investor should know before investing in NMFC’s common stock. Please read this prospectus supplement and the accompanying prospectus before investing and keep it for future reference. NMFC and the Operating Company file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission (http://www.sec.gov), which is available free of charge by contacting NMFC 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.

(1) All expenses of the offering, including the sales load, will be borne by the Operating Company. The Operating Company will incur approximately $[            ] of estimated expenses, excluding the sales load, in connection with this offering. Stockholders will indirectly bear such expenses, including the sales load, through NMFC’s ownership of common membership units of the Operating Company.

(2) To the extent that the underwriters sell more than [            ] shares of NMFC’s common stock, the underwriters have the option to purchase up to an additional [            ] shares of NMFC’s common stock at the public offering price, less the sales load, within [ ] days of the date of this prospectus. If the underwriters exercise this option in full, the total public offering price, sales load and proceeds to us will be $            , $ and $            , respectively. If the underwriters exercise their option to purchase additional shares of NMFC’s common stock, NMFC will use the proceeds from the exercise of this option to purchase additional common membership units of the Operating Company.

The underwriters expect to deliver the shares against payment in New York, New York on or about             , 20     .

            , 20    

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT

PROSPECTUS

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, any shares of our common stock 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 our common stock. 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 of common stock 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” and in the “Summary” and “Risk Factors” sections before you make an investment decision.

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FEES AND EXPENSES

The following table is intended to assist you in understanding the costs and expenses that you will bear directly or indirectly. We caution you that some of the percentages indicated in the table below are estimates and may vary. Except where the context suggests otherwise, whenever this prospectus supplement and the accompanying prospectus contains a reference to fees or expenses paid by “you”, “NMFC”, the “Operating Company”, or “us” or that “we”, “NMFC”, or the “Operating Company” will pay fees or expenses, stockholders will indirectly bear such fees or expenses through NMFC’s investment in the Operating Company.

Example

The following example, required by the SEC, demonstrates the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in NMFC’s common stock. In calculating the following expense amounts, we have assumed that our borrowings and annual operating expenses would remain at the levels set forth in the table above. See Note 6 below for additional information regarding certain assumptions regarding our level of leverage.

The example and the expenses in the tables above should not be considered a representation of future expenses, and actual expenses may be greater or less than those shown.

While the example assumes, as required by the applicable rules of the SEC, a 5.0% annual return, our performance will vary and may result in a return greater or less than 5.0%. The incentive fee under the Investment Management Agreement, which, assuming a 5% annual return, would either not be payable or would have an insignificant impact on the expense amounts shown above, is not included in the example. This illustration assumes that we will not realize any capital gains (computed net of all realized capital losses and unrealized capital depreciation) in any of the indicated time periods. If we achieve sufficient returns on our investments, including through the realization of capital gains, to trigger an incentive fee of a material amount, our expenses and returns to our investors would be higher. For example, if we assumed that we received our 5% annual return completely in the form of net realized capital gains on our investments, computed net of all cumulative unrealized depreciation on our investments, the projected dollar amount of total cumulative expenses set forth in the above illustration would be as follows:

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The example assumes no sales load. In addition, while the examples assume reinvestment of all distributions at net asset value, participants in NMFC’s dividend reinvestment plan will receive a number of shares of NMFC’s common stock determined by dividing the total dollar amount of the distribution payable to a participant by the market price per share of NMFC’s common stock at the close of trading on the dividend payment date. The market price per share of NMFC’s common stock may be at, above or below net asset value. See “Dividend Reinvestment Plan” in the accompanying prospectus for additional information regarding the dividend reinvestment plan.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement contains 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, the Operating Company’s current and prospective portfolio investments, our industry, our beliefs, and our assumptions. Words such as “anticipates,” “expects,” “intends,” “plans,” “believes,” “seeks,” “estimates,” “would,” “should,” “targets,” “projects,” and variations of these words and similar expressions are intended to identify forward-looking statements. The forward-looking statements contained in this prospectus supplement involve risks and uncertainties, including statements as to:

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:

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 the Operating

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Company’s 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 should not be regarded as a representation by us that our plans and objectives will be achieved. These risks and uncertainties include those described or identified in “Risk Factors” 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 of 1933, as amended, or the Securities Act.

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CAPITALIZATION

The following table sets forth our capitalization as of [            ], 20[    ]:

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

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USE OF PROCEEDS

We estimate that we will receive net proceeds from the sale of the          shares of NMFC’s common stock in this offering of approximately $         million, or approximately $         million if the underwriters exercise their option to purchase additional shares in full (in each case using the last reported closing price of NMFC’s common stock on                 , 20 of $     per share), after deducting estimated offering expenses of approximately $ payable by the Operating Company.

The Operating Company intends to use the net proceeds from this offering for new investments in portfolio companies in accordance with the Operating Company’s investment objective and strategies described in this prospectus supplement and the accompanying prospectus, to temporarily repay indebtedness (which will be subject to reborrowing), to pay our operating expenses, to pay distributions to our stockholders/unit holders and for general corporate purposes. The Operating Company is continuously identifying, reviewing and, to the extent consistent with its investment objective, funding new investments. As a result, we typically raise capital as we deem appropriate to fund such new investments.

We estimate that it will take      to          months for the Operating Company to substantially invest the net proceeds of this offering, depending on the availability of attractive opportunities and market conditions. However, we can offer no assurance that we will be able to achieve this goal.

Proceeds not immediately used for new investments or the temporary repayment of debt will be invested primarily in cash, cash equivalents, U.S. government securities and other high-quality investments that mature in one year or less from the date of investment. These temporary investments are expected to provide a lower net return than we hope to achieve from the Operating Company’s target investments.

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UNDERWRITING

NMFC, the Operating Company and the underwriters named below have entered into an underwriting agreement with respect to the shares being offered. Subject to certain conditions, each underwriter has severally agreed to purchase the number of shares indicated in the following table. [        ] are the representatives of the underwriters.

The underwriters are committed to take and pay for all of the shares being offered, if any are taken, other than the shares covered by the option described below unless and until this option is exercised.

If the underwriters sell more shares than the total number set forth in the table above, the underwriters have an option to buy up to an additional [        ] shares from NMFC. They may exercise that option for [    ] days. If any shares are purchased pursuant to this option, the underwriters will severally purchase shares in approximately the same proportion as set forth in the table above.

The following table shows the per share and total underwriting discounts and commissions (sales load) to be paid to the underwriters by the Operating Company. Such amounts are shown assuming both no exercise and full exercise of the underwriters’ option to purchase [        ] additional shares. This offering will conform with the requirements set forth in Financial Industry Regulatory Authority Rule 2310. In compliance with such requirements, the underwriting discounts and commissions in connection with the sale of securities will not exceed 10% of gross proceeds of this offering.

Shares sold by the underwriters to the public will initially be offered at the public offering price set forth on the cover of this prospectus supplement. Any shares sold by the underwriters to securities dealers may be sold at a discount of up to $         per share from the public offering price. If all the shares are not sold at the initial offering price, the representatives may change the public offering price and the other selling terms. The offering of the shares by the underwriters is subject to receipt and acceptance and subject to the underwriters’ right to reject any order in whole or in part.

NMFC, each of its officers and directors, and each of the members of the Investment Adviser’s investment committee have agreed with the underwriters, subject to certain exceptions, not to dispose of or hedge any shares of NMFC’s common stock or securities convertible into or exchangeable for shares of NMFC’s common stock during the period from the date of this prospectus supplement continuing through the date [    ] days after the date of this prospectus supplement, except with the prior written consent of [            ].

The 180-day restricted period described in the preceding paragraph will be automatically extended if: (1) during the last 17 days of the 180-day restricted period the NMFC issues an earnings release or announce material news or a material event; or (2) prior to the expiration of the 180-day restricted period, the NMFC

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announces that it will release earnings results during the 15-day period following the last day of the 180-day period, in which case the restrictions described in the preceding paragraph will continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release of the announcement of the material news or material event.

NMFC’s common stock is listed on the New York Stock Exchange under the symbol “NMFC”.

In connection with the offering, the underwriters may purchase and sell shares of common stock in the open market. These transactions may include short sales, stabilizing transactions and purchases to cover positions created by short sales. Short sales involve the sale by the underwriters of a greater number of shares than they are required to purchase in the offering. “Covered” short sales are sales made in an amount not greater than the underwriters’ option to purchase additional shares from us in the offering. The underwriters may close out any covered short position by either exercising their option to purchase additional shares or purchasing shares in the open market. In determining the source of shares to close out the covered short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase additional shares pursuant to the option granted to them. “Naked” short sales are any sales in excess of such option. The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the common stock in the open market after pricing that could adversely affect investors who purchase in the offering. Stabilizing transactions consist of various bids for or purchases of common stock made by the underwriters in the open market prior to the completion of the offering.

The underwriters may also impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the representatives have repurchased shares sold by or for the account of such underwriter in stabilizing or short covering transactions.

Purchases to cover a short position and stabilizing transactions, as well as other purchases by the underwriters for their own account, may have the effect of preventing or retarding a decline in the market price of the company’s stock, and together with the imposition of the penalty bid, may stabilize, maintain or otherwise affect the market price of the common stock. As a result, the price of the common stock may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued at any time. These transactions may be effected on the New York Stock Exchange, in the over-the-counter market or otherwise.

The underwriters do not expect sales to discretionary accounts to exceed five percent of the total number of shares offered.

We estimate that our share of the total expenses of the offering, excluding underwriting discounts and commissions, will be approximately $[            ]. The Operating Company will pay all of the expenses incurred by us in connection with this offering.

NMFC and the Operating Company have agreed to indemnify the several underwriters against certain liabilities, including liabilities under the Securities Act of 1933.

The underwriters and their respective affiliates are full-service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. Certain of the underwriters and their respective affiliates may, from time to time, perform various financial advisory and investment banking services for the company, for which they will receive customary fees and expenses.

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In the ordinary course of their various business activities, the underwriters and their respective affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers and such investment and securities activities may involve securities and/or instruments of the issuer. The underwriters and their respective affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

The principal business address of [            ], [            ], [            ], [    ] [            ].

Each of the underwriters may arrange to sell common shares offered hereby in certain jurisdictions outside the United States, either directly or through affiliates, where they are permitted to do so.

[Describe any other specific transactions and compensation related thereto to the extent required to be disclosed by applicable law or regulation.]

[Insert applicable legends for jurisdictions in which offers and sales may be made.]

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LEGAL MATTERS

Certain legal matters in connection with the securities offered hereby will be passed upon for us by Sutherland Asbill & Brennan LLP, Washington, District of Columbia. Certain legal matters in connection with the securities offered hereby will be passed upon for the underwriters by             ,             ,             .

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

With respect to the unaudited interim financial information of New Mountain Finance Holdings, L.L.C. as of March 31, 2012 and for the periods ended March 31, 2012 and 2011, and of New Mountain Finance Corporation and New Mountain Finance AIV Holdings Corporation as of March 31, 2012 and for the three months then ended, which is included in this prospectus, Deloitte & Touche LLP, an independent registered public accounting firm, has applied limited procedures in accordance with the standards of the Public Company Accounting Oversight Board (United States) for a review of such information. However, as stated in their reports included in this prospectus, they did not audit and they do not express an opinion on that interim financial information. Accordingly, the degree of reliance on their reports on such information should be restricted in light of the limited nature of the review procedures applied. Deloitte & Touche LLP are not subject to the liability provisions of Section 11 of the Securities Act of 1933 for their reports on the unaudited interim financial information because those reports are not “reports” or a “part” of the Registration Statement prepared or certified by an accountant within the meaning of Sections 7 and 11 of the Act.

The financial statements of New Mountain Finance Holdings, L.L.C. as of December 31, 2011 and 2010 and for each of the three years ended December 31, 2011, and the financial statements of New Mountain Finance Corporation and New Mountain Finance AIV Holdings Corporation as of December 31, 2011 and for the period from May 19, 2011 (Commencement of Operations) to December 31, 2011, including the Senior Securities table included in the accompanying prospectus have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports appearing herein and elsewhere in the Registration Statement. Such financial statements and information included in the Senior Securities table as of December 31, 2011, 2010 and 2009 have been so included in reliance upon the reports of such firm, given their authority as experts in accounting and auditing.

The principal business address of Deloitte & Touche LLP is 2 World Financial Center, New York, New York 10281.

AVAILABLE INFORMATION

We have filed with the SEC a registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to the shares of common stock offered by this prospectus supplement and the accompanying prospectus. The registration statement contains additional information about us and the shares of common stock being offered by this prospectus supplement and the accompanying prospectus.

We are required to file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Exchange Act. You may inspect and copy these reports, proxy statements and other information, as well as the registration statement and related exhibits and schedules, at the Public Reference Room of the SEC at 100 F Street, N.E., Washington, District of Columbia 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an internet site that contains reports, proxy and information statements and other information filed electronically by us with the SEC, which are available on the SEC’s website at http://www.sec.gov. Copies of these reports, proxy and information statements and other information may be

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obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC’s Public Reference Section, 100 F Street, N.E., Washington, District of Columbia 20549. This information will also be available free of charge by contacting us at 787 Seventh Avenue, 48th Floor, New York, New York 10019, by telephone at (212) 720-0300, or on our website at http://www.newmountainfinance.com. Information contained on our website or on the SEC’s web site about us is not incorporated into this prospectus supplement and the accompanying prospectus and you should not consider information contained on our website or on the SEC’s website to be part of this prospectus supplement and the accompanying prospectus.

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[            ] Shares

New Mountain Finance Corporation

Common Stock

PRELIMINARY PROSPECTUS SUPPLEMENT

[Underwriters]