UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

New Mountain Finance Corporation

(Name of Issuer)

Common Stock

(Title of Class of Securities)

647551100

(CUSIP Number)

December 31, 2012

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

¨ Rule 13d-1(c)

x Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Item 1(a) Name of issuer: New Mountain Finance Corporation

Item 1(b) Address of issuer’s principal executive offices:

787 Seventh Avenue, 48^th Floor, New York, NY 10019

2(a) Name of person filing:

2(b) Address or principal business office or, if none, residence:

For all filing persons listed in 2(a) above: One North Wacker Drive, Suite 2200, Chicago, Illinois 60606

2(c) Citizenship:

2(d) Title of class of securities:

Common Stock

2(e) CUSIP No.:

647551100

Item 3. If this statement is filed pursuant to §§240.13d–1(b) or 240.13d–2(b) or (c), check whether the person filing is a:

NOT APPLICABLE

Item 4. Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

Instruction. For computations regarding securities which represent a right to acquire an underlying security see §240.13d–3(d)(1).

Item 5. Ownership of 5 Percent or Less of a Class. If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than 5 percent of the class of securities, check the following [X].

Instruction. Dissolution of a group requires a response to this item.

Item 6. Ownership of More than 5 Percent on Behalf of Another Person. If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required.

NOT APPLICABLE

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person. If a parent holding company or control person has filed this schedule pursuant to Rule 13d–1(b)(1)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company or control person has filed this schedule pursuant to Rule 13d–1(c) or Rule 13d–1(d), attach an exhibit stating the identification of the relevant subsidiary.

NOT APPLICABLE

Item 8. Identification and Classification of Members of the Group

If a group has filed this schedule pursuant to §240.13d–1(b)(1)(ii)(J), so indicate under Item 3(j) and attach an exhibit stating the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to Rule 13d–1(c) or Rule 13d–1(d), attach an exhibit stating the identity of each member of the group.

Prior to disposing of such shares, Adams Street 2007 Direct Fund, L.P. (“AS 2007”), Adams Street 2008 Direct Fund, L.P. (“AS 2008”), Adams Street 2009 Direct Fund, L.P. (“AS 2009”), Adams Street Direct Co-Investment Fund, L.P. (“AS Co-Investment”), and Adams Street Co-Investment Fund II, L.P. (“AS Co-Investment II”) were the record owners of Common Stock of the Issuer, which shares may have been deemed beneficially owned by Adams Street Partners, LLC, the managing member of each of AS 2007’s, AS 2008’s, AS 2009’s, AS Co-Investment’s, and AS Co-Investment II’s respective general partners.

Item 9. Notice of Dissolution of Group. Notice of dissolution of a group may be furnished as an exhibit stating the date of the dissolution and that all further filings with respect to transactions in the security reported on will be filed, if required, by members of the group, in their individual capacity. See Item 5.

NOT APPLICABLE

Item 10. Certifications

NOT APPLICABLE

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: February 14, 2013

EXHIBIT 1

AGREEMENT TO MAKE A JOINT FILING

The undersigned hereby agree that this Schedule 13G is filed by and on behalf of each of them.

Date: February 14, 2013