Exhibit (l)(2)

[Letterhead of Sutherland Asbill & Brennan LLP]

July 12, 2012

New Mountain Finance Corporation

787 Seventh Avenue, 48^th Floor

New York, NY 10019

Ladies and Gentlemen:

We have acted as counsel to New Mountain Finance Corporation, a Delaware corporation (the “Company”), in connection with the preparation and filing by the Company with the Securities and Exchange Commission (the “Commission”) of a registration statement on Form N-2 on April 12, 2012 (as amended from time to time, the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act”), with respect to the offer, issuance and sale from time to time pursuant to Rule 415 under the Securities Act of shares of the Company’s common stock, par value $0.01 per share (the “Common Stock”). The Registration Statement provides that the Common Stock may be issued from time to time in amounts, at prices, and on terms to be set forth in one or more supplements to the final prospectus included in the Registration Statement at the time it becomes effective.

This opinion letter is rendered in connection with the issuance and sale of up to 6,037,500 shares of the Company’s Common Stock (the “Shares”), as described in the prospectus supplement, dated as of July 12, 2012, filed with the Commission pursuant to Rule 497 under the Securities Act (the “Prospectus Supplement”). The Shares are being sold by the Company pursuant to an underwriting agreement, dated as of July 12, 2012, by and among the Company, New Mountain Finance Holdings, L.L.C., New Mountain Finance Advisers BDC, L.L.C., New Mountain Finance Administration L.L.C., and Goldman, Sachs & Co., Wells Fargo Securities, LLC and Morgan Stanley & Co. LLC, as representatives of the several underwriters named therein (the “Underwriting Agreement”).

As counsel to the Company, we have participated in the preparation of the Registration Statement and the Prospectus Supplement and have examined the originals or copies, certified or otherwise identified to our satisfaction as being true copies, of the following:

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July 12, 2012

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With respect to such examination and our opinion expressed herein, we have assumed, without any independent investigation or verification, (i) the genuineness of all signatures on all documents submitted to us for examination, (ii) the legal capacity of all natural persons, (iii) the authenticity of all documents submitted to us as originals, (iv) the conformity to original documents of all documents submitted to us as conformed or reproduced copies and the authenticity of the originals of such copied documents, and (v) that all certificates issued by public officials have been properly issued. We also have assumed without independent investigation or verification the accuracy and completeness of all corporate records made available to us by the Company.

As to certain matters of fact relevant to the opinions in this opinion letter, we have relied upon certificates of public officials (which we have assumed remain accurate as of the date of this opinion), upon certificates and/or representations of officers and employees of the Company, upon such other certificates as we deemed appropriate, and upon such other data as we have deemed to be appropriate under the circumstances. We have not independently established the facts, or in the case of certificates of public officials, the other statements, so relied upon.

The opinions set forth below are limited to the effect of the Delaware General Corporation Law (the “DGCL”), as in effect on the date hereof, and we express no opinion as to the applicability or effect of any other laws of such jurisdiction or the laws of any other jurisdictions. Without limiting the preceding sentence, we express no opinion as to any state securities or broker-dealer laws or regulations thereunder relating to the offer, issuance and sale of the Shares.

On the basis of and subject to the foregoing, and in reliance thereon, and subject to the limitations and qualifications set forth in this opinion letter, we are of the opinion that the Shares have been duly authorized for issuance and, when issued and paid for in accordance with the terms and conditions of the Underwriting Agreement, will be validly issued, fully paid and nonassessable.

The opinions expressed in this opinion letter (i) are strictly limited to the matters stated in this opinion letter, and without limiting the foregoing, no other opinions are to be implied and (ii) are only as of the date of this opinion letter, and we are under no obligation, and do not

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July 12, 2012

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undertake, to advise the addressee of this opinion letter or any other person or entity either of any change of law or fact that occurs, or of any fact that comes to our attention, after the date of this opinion letter, even though such change or such fact may affect the legal analysis or a legal conclusion in this opinion letter.

We hereby consent to the filing of this opinion letter as an exhibit to the Registration Statement. We do not admit by giving this consent that we are in the category of persons whose consent is required under Section 7 of the Securities Act. Otherwise, this opinion letter may not be relied on by, or furnished to, any other person or entity without our prior written consent and without limiting the foregoing, may not be quoted, published or otherwise disseminated, without in each instance our prior written consent.