New Mountain Finance Corp

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of

the Securities Exchange Act of 1934 (Amendment No. )

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

New Mountain Finance Corporation

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

Dear Stockholder:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders (“Annual Meeting”) of New Mountain Finance Corporation (“NMFC” or the “Company”) to be held on May 12, 2026 at 10:30 a.m., Eastern Time. You will be able to participate in the Annual Meeting, vote and submit your questions via live webcast by visitingwww.virtualshareholdermeeting.com/NMFC2026. Prior to the Annual Meeting you will be able to vote electronically atwww.proxyvote.com.

The notice of the Annual Meeting and the proxy statement provide an outline of the business to be conducted at the Annual Meeting. At the Annual Meeting, the stockholders of NMFC will be asked to: (i) elect three directors to the board of directors of NMFC to serve for a term of three years, or until their respective successors are duly elected and qualify; (ii) ratify the appointment of Deloitte & Touche LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2026; and (iii) transact such other business that may properly come before the Annual Meeting.

It is important that your shares be represented at the Annual Meeting. If you are unable to participate in the Annual Meeting during the scheduled time, I urge you to follow the instructions on the Notice of Internet Availability of Proxy Materials to vote your proxy on the Internet. I encourage you to vote via the Internet, as it saves us significant time and processing costs. However, on the Notice of Internet Availability of Proxy Materials, you will also find instructions on how to request a hard copy of the Proxy Statement and proxy card free of charge and you may vote your proxy by returning your proxy card to us after you request the hard copy materials. Your vote is important.

To vote or to submit your questions during the Annual Meeting, please log on towww.virtualshareholdermeeting.com/NMFC2026. You will need to enter the 16-digit control number on your notice of the Annual Meeting. Your vote is important to us.

Sincerely yours,

John R. Kline

President and Chief Executive Officer

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

AND INTERNET AVAILABILITY OF PROXY MATERIALS

TO BE HELD ON MAY 12, 2026

To the Stockholders of New Mountain Finance Corporation:

A meeting (the “Annual Meeting”) of stockholders of New Mountain Finance Corporation (“NMFC” or the “Company”) will be held on May 12, 2026 at 10:30 a.m., Eastern Time. We are very pleased that this year’s Annual Meeting will be a completely virtual meeting of stockholders, which will be conducted via live webcast. It is important to note that stockholders have the same rights and opportunities by participating in a virtual meeting, as they would if attending an in-person meeting. You can participate in the Annual Meeting, vote and submit your questions during the Annual Meeting by visiting www.virtualshareholdermeeting.com/NMFC2026. You must have your 16-digit control number in order to access the Annual Meeting. The Annual Meeting will be held for the following purposes:

You have the right to receive notice of, and to vote at, the Annual Meeting if you were a stockholder of record at the close of business on March 13, 2026. NMFC is furnishing the Proxy Statement and proxy card to its stockholders on the Internet, rather than mailing printed copies of those materials to each stockholder. If you received a Notice of Internet Availability of Proxy Materials by mail, you will not receive a printed copy of the Proxy Statement and proxy card unless you request them. Instead, the Notice of Internet Availability of Proxy Materials will instruct you as to how you may access and review the Proxy Statement, and vote your proxy, on the Internet.

Your vote is extremely important to us. If you are unable to attend the Annual Meeting, we encourage you to vote your proxy on the Internet by following the instructions provided on the Notice of Internet Availability of Proxy Materials. You may also request from us, free of charge, hard copies of the Proxy Statement and proxy card for the Company by following the instructions on the Notice of Internet Availability of Proxy Materials. In the event there are not sufficient votes for a quorum or to approve NMFC’s proposals at the time of the Annual Meeting, the Annual Meeting may be adjourned in order to permit further solicitation of proxies by NMFC.

By Order of the board of directors,

Eric Kane Corporate Secretary

New York, New York

This is an important meeting. To ensure proper representation at the Annual Meeting, please follow the instructions on the Notice of Internet Availability of Proxy Materials to vote your proxy via the Internet or request, complete, sign, date and return a proxy card. Even if you vote your shares prior to the Annual Meeting, you still may attend the Annual Meeting and vote your shares via the Internet.

New York, NY 10019

PROXY STATEMENT

2026 Annual Meeting

of Stockholders of New Mountain Finance Corporation

This Proxy Statement is furnished in connection with the solicitation of proxies by the board of directors of New Mountain Finance Corporation (“NMFC”, the “Company”, “we”, “us” or “our”) for use at the NMFC 2026 Annual Meeting (the “Annual Meeting”) to be held on May 12, 2026, at 10:30 a.m., Eastern Time. You can virtually attend the Annual Meeting online, vote your shares electronically and submit questions during the Annual Meeting by visiting www.virtualshareholdermeeting.com/NMFC2026, and at any postponements or adjournments thereof. You will need to enter the 16-digit control number on your notice of the Annual Meeting. It is important to note that shareholders have the same rights and opportunities by participating in a virtual meeting, as they would if attending an in-person meeting. This Proxy Statement is being provided to the stockholders of NMFC via the Internet on or about March 31, 2026. In addition, a Notice of Annual Meeting and a Notice of Internet Availability of Proxy Materials are being sent to our stockholders of record on or about March 31, 2026.

We encourage you to vote your shares of NMFC’s stock either by voting via the Internet while virtually attending the Annual Meeting, by telephone, or by granting a proxy(i.e., authorizing someone to vote your shares). Shares represented by duly executed proxies will be voted in accordance with your instructions. If you give no instructions on the proxy card, the shares covered by the proxy card will be voted “FOR” the matters listed in this Proxy Statement.

If you are a stockholder “of record”(i.e., you hold shares directly in your name), you may revoke a proxy at any time before it is exercised by notifying the proxy tabulator, Broadridge Shareholder Solutions, Inc., in writing, by submitting a properly executed, later-dated proxy, or by voting virtually at the Annual Meeting. Please send your notification to Vote Processing, c/o Broadridge, 51 Mercedes Way, Edgewood, NY 11717, and submit a properly executed, later-dated proxy or vote virtually at the Annual Meeting. Any stockholder of record attending the Annual Meeting may vote virtually whether or not he, she or it has previously voted his, her or its shares. If your shares are held for your account by a broker, bank or other institution or nominee (“Broker Securities”), you may vote such securities at the Annual Meeting only if you obtain proper written authority from your institution or nominee and present it at the Annual Meeting. All of our directors are encouraged to attend the Annual Meeting. Stockholders have no dissenters’ or appraisal rights in connection with any of the proposals described herein.

Stockholders of record may also vote either via the Internet or by telephone. Specific instructions to be followed by stockholders of record interested in voting via the Internet or the telephone are shown on the proxy card. The Internet and telephone voting procedures are designed to authenticate the voter’s identity and to allow stockholders to vote their shares and confirm that their instructions have been properly recorded.

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