<PAGE>

                                                                    EXHIBIT 10.1

                              FIRST AMENDMENT TO
                              ------------------
                  FOURTH AMENDED AND RESTATED CREDIT AGREEMENT
                  --------------------------------------------
                        AND SERVICING SECURITY AGREEMENT
                        --------------------------------


     THIS FIRST AMENDMENT TO FOURTH AMENDED AND RESTATED CREDIT AGREEMENT AND
SERVICING SECURITY AGREEMENT (the "Amendment") dated as of August 31, 1999 by
and among NEW CENTURY MORTGAGE CORPORATION, a California corporation (the
"Company"), the lenders party to the Credit Agreement referred to below
(collectively, the "Lenders" and individually, a "Lender") and U.S. BANK
NATIONAL ASSOCIATION, a national banking association, in its capacity as agent
for the Lenders (in such capacity, together with any successor agents appointed
thereunder, the "Agent").

     WITNESSETH THAT:

     WHEREAS, the Company, the Lenders and the Agent are parties to a Fourth
Amended and Restated Credit Agreement dated as of May 26, 1999 (the "Credit
Agreement"), pursuant to which the Lenders provide the Company with a revolving
mortgage warehousing credit facility;

     WHEREAS, the Company and the Agent are parties to a Servicing Security
Agreement dated as of May 29, 1998 (the "Servicing Security Agreement"); and

     WHEREAS, the Company and the Lenders have agreed to amend the Credit
Agreement and Servicing Security Agreement upon the terms and conditions herein
set forth;

     NOW, THEREFORE, for value received, the receipt and sufficiency of which
are hereby acknowledged, the Company and the Lenders agree as follows:

     1.   Certain Defined Terms.  Each capitalized term used herein without
          ---------------------
being defined herein that is defined in the Credit Agreement shall have the
meaning given to it therein.

     2.   Amendments to Credit Agreement.  The Credit Agreement is hereby
          ------------------------------
amended as follows:

          (a) Section 4.09(f) of the Credit Agreement is hereby amended in its
     entirety to read as follows:

               (f)  Liens incurred in connection with gestation repurchase
               agreements or similar arrangements, including, without
               limitation,
<PAGE>

               (i) arrangements under which NCFC or its Subsidiaries are
               required to repurchase Mortgage-backed Securities or Mortgage
               Loans from any Lender or other counterparty reasonably
               satisfactory to the Agent, or (ii) credit facilities structured
               as loan and security agreements; provided, that (x) such
               gestation repurchase agreements or similar arrangements are not
               used to fund wet Mortgage Loans, and (y) such gestation
               repurchase agreements or similar arrangements are entered into in
               the ordinary course of business in contemplation of the
               subsequent non-recourse sale of such Mortgage-backed Securities
               or Mortgage Loans;

          (b)  Section 4.09 is further amended by deleting "and" at the end of
     subsection (h) thereof, deleting the period at the end of subsection (i)
     thereof and inserting "; and" therefor, and inserting at the end thereof a
     new subsection (j) to read as follows:

               (j) a pledge of the stock of NC Residual II Corporation to
               Financial Securities Assurance Corporation.

          (c)  Section 4.11(c) of the Credit Agreement is hereby amended in its
     entirety to read as follows:

               (c) Guarantees by NCFC of the Company's and NCCC's obligations
               relating to (i) Indebtedness permitted by Sections 4.08(d) and
               (g), or (ii) the Strategic Alliance Agreement described in
               Section 4.10(j); and

          (d)  Section 4.13 of the Credit Agreement is hereby amended in its
     entirety to read as follows:

                    4.13  Restricted Payments.  The Company and NCFC will not
                          -------------------
               make any Restricted Payments, other than (a) dividends paid by
               NCFC on its 1998A Convertible Preferred Stock and its 1999A
               Convertible Preferred Stock in an aggregate amount not to exceed
               $3,000,000 per annum, (b) dividends paid by the Company to NCFC
               to enable NCFC to pay such dividends in an amount not to exceed
               $3,000,000 per annum, and (c) repurchases by NCFC of shares of
               its common stock pursuant to the Stock Repurchase Program, in
               each case provided that, both before and after giving effect to
               such dividend payments or repurchases, the Borrower and NCFC are
               in compliance with the covenants set forth in Section 4 of

                                      -2-
<PAGE>

               this Agreement and no Event of Default or Unmatured Event of
               Default has occurred and is continuing.

          (e)  Exhibit E to the Credit Agreement is hereby amended in its
     entirety to read as set forth on Exhibit E hereto.

          (f)  Schedule 1.01(b) to the Credit Agreement is hereby amended in its
     entirety to read as set forth on Schedule 1.01(b) hereto.

     3.   Amendments to the Servicing Security Agreement.  The Servicing
          ----------------------------------------------
Security Agreement is hereby amended as follows:

          (a)  The definitions of "Servicing Contracts" and "Servicing Rights"
     are hereby amended in their entireties to read as follows:

               "Servicing Contracts" shall mean any and all contracts or
                -------------------
               agreements purchased by the Company or entered into by the
               Company for its own account (and not as nominee or subservicer),
               whether now existing or hereafter purchased or entered into,
               pursuant to which the Company services Mortgage Loans or Mortgage
               Loan pools for others (other than NCCC).

               "Servicing Rights" shall mean any and all rights of the Company
                ----------------
               held for its own account (and not as nominee or subservicer),
               whether pursuant to a Servicing Contract or otherwise, to service
               Mortgage Loans or Mortgage Loan pools for others (other than
               NCCC), including, without limitation, (i) all rights to collect
               payments due and enforce the rights of the mortgagee under any
               Mortgage Loans, (ii) all rights to receive compensation and
               termination fees under any Servicing Contract and (iii) all
               rights to receive the proceeds from any sale or other transfer of
               the Company's interest in any Servicing Contract.

          (b)  Section 2(d) of the Servicing Security Agreement is hereby
     amended in its entirety to read as follows:

               (d)  all books, correspondence, credit files, records, invoices,
               bills of lading, and other documents, including, without
               limitation, all tapes, cards, computer runs, and other papers and
               documents in the possession or control of the Company or any
               computer bureau from time to time acting for the Company, but
               specifically excluding books, correspondence, credit files,
               records, invoices,

                                      -3-
<PAGE>

               bills of lading, and other documents relating to Mortgage Loans
               pledge to third parties to secure Indebtedness permitted by
               Section 4.08(g) of the Credit Agreement;

     4.   Conditions to Effectiveness of this Amendment.  This Amendment shall
          ---------------------------------------------
become effective when the Agent shall have received at least thirteen (13)
counterparts of this Amendment, duly executed by the Company and the Required
Lenders, provided the following conditions are satisfied:

          (a) Before and after giving effect to this Amendment, the
     representations and warranties of the Company in Section 3 of the Credit
     Agreement, Section 5 of the Pledge and Security Agreement and Section 4 of
     the Servicing Security Agreement, of NCFC in Section 15 of the Guaranty,
     and of NCCC in Section 15 of the NCCC Guaranty shall be true and correct as
     though made on the date hereof, except for changes that are permitted by
     the terms of the Credit Agreement.

          (b) Before and after giving effect to this Amendment, no Event of
     Default and no Unmatured Event of Default shall have occurred and be
     continuing.

          (c) No material adverse change in the business, assets, financial
     condition or prospects of the Company or NCFC shall have occurred since May
     26, 1999.

          (d) The Agent shall have received the following, each duly executed or
     certified, as the case may be, and dated as of the date of delivery
     thereof:

               (i)   copy of resolutions of the Board of Directors of the
               Company, certified by its respective Secretary or Assistant
               Secretary, authorizing or ratifying the execution, delivery and
               performance of this Amendment;

               (ii)  a certified copy of any amendment or restatement of the
               Articles of Incorporation or the By-laws of the Company made or
               entered following the date of the most recent certified copies
               thereof furnished to the Lenders;

               (iii) certified copies of all documents evidencing any necessary
               corporate action, consent or governmental or regulatory approval
               (if any) with respect to this Amendment;

                                      -4-
<PAGE>

               (iv) a Reaffirmation of NCFC Guaranty duly executed by NCFC;

               (v)  a Reaffirmation of NCCC Guaranty duly executed by NCCC; and

               (vi) such other documents, instruments, opinions and approvals
               as the Agent may reasonably request.

          (e)  The Agent shall have received the amendment fee required by
     Section 8.16 of the Credit Agreement.

     5.   Acknowledgments.  The Company and each Lender acknowledge that, as
          ---------------
amended hereby, the Credit Agreement remains in full force and effect with
respect to the Company and the Lenders, and that each reference to the Credit
Agreement in the Loan Documents shall refer to the Credit Agreement as amended
hereby.  The Company confirms and acknowledges that it will continue to comply
with the covenants set out in the Credit Agreement and the other Loan Documents,
as amended hereby, and that its representations and warranties set out in the
Credit Agreement and the other Loan Documents, as amended hereby, are true and
correct as of the date of this Amendment.  The Company represents and warrants
that (i) the execution, delivery and performance of this Amendment is within its
corporate powers and has been duly authorized by all necessary corporate action;
(ii) this Amendment has been duly executed and delivered by the Company and
constitutes the legal, valid and binding obligation of the Company, enforceable
against the Company in accordance with its terms (subject to limitations as to
enforceability which might result from bankruptcy, insolvency, or other similar
laws affecting creditors' rights generally and general principles of equity) and
(iii) no Events of Default or Unmatured Events of Default exist.

     6.   General.
          -------

          (a) The Company agrees to reimburse the Agent upon demand for all
     reasonable expenses (including reasonable attorneys fees and legal
     expenses) incurred by the Agent in the preparation, negotiation and
     execution of this Amendment and any other document required to be furnished
     herewith, and to pay and save the Lenders harmless from all liability for
     any stamp or other taxes which may be payable with respect to the execution
     or delivery of this Amendment, which obligations of the Company shall
     survive any termination of the Credit Agreement.

          (b) This Amendment may be executed in as many counterparts as may be
     deemed necessary or convenient, and by the different parties hereto on

                                      -5-
<PAGE>

     separate counterparts, each of which, when so executed, shall be deemed an
     original but all such counterparts shall constitute but one and the same
     instrument.

          (c) Any provision of this Amendment which is prohibited or
     unenforceable in any jurisdiction shall, as to such jurisdiction, be
     ineffective to the extent of such prohibition or unenforceability without
     invalidating the remaining portions hereof or affecting the validity or
     enforceability of such provisions in any other jurisdiction.

          (d) This Amendment shall be governed by, and construed in accordance
     with, the internal law, and not the law of conflicts, of the State of
     Minnesota, but giving effect to federal laws applicable to national banks.

          (e) This Amendment shall be binding upon the Company, the Lenders, the
     Agent and their respective successors and assigns, and shall inure to the
     benefit of the Company, the Lenders, the Agent and the successors and
     assigns of the Lenders and the Agent.

                                      -6-
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be executed as of the day and year first above written.


                              NEW CENTURY MORTGAGE
                              CORPORATION


                              By  /s/ Brad A. Morrice
                                 -------------------------------------------
                                 Its  Chairman, CEO
                                     ---------------------------------------




                              U.S. BANK NATIONAL ASSOCIATION,

                              By  /s/ Edwin D. Jenkens
                                 -----------------------------------------
                                 Its  Senior Vice President
                                    --------------------------------------



                              GUARANTY FEDERAL BANK, FSB

                              By  /s/ W. James Meintjes
                                 ------------------------------------------
                                 Its  Vice President
                                    ---------------------------------------




                    [Signature Page for First Amendment to
                 Fourth Amended and Restated Credit Agreement]
<PAGE>

                                         BANK UNITED


                                         By  /s/ Michele Perrin
                                            ------------------------------------
                                            Its  VP. Mtg. Banker Finance
                                                --------------------------------


                                         RESIDENTIAL FUNDING CORPORATION


                                         By  /s/ ^^ILLEGIBLE SIGNATURE^^
                                            ------------------------------------
                                            Its  Director
                                                --------------------------------


                                         BANK ONE, TEXAS, N.A.


                                         By  /s/ Gregory T. Martin
                                            ------------------------------------
                                            Its  Assistant Vice President
                                                --------------------------------


                    [Signature Page for First Amendment to
                 Fourth Amended and Restated Credit Agreement]
<PAGE>

                                         UNION BANK OF CALIFORNIA, N.A.


                                         By  /s/ Robin Courtney
                                            ------------------------------------
                                            Its  Investment Banking Officer
                                                --------------------------------



                    [Signature Page for First Amendment to
                 Fourth Amended and Restated Credit Agreement]
<PAGE>

                         REAFFIRMATION OF NCFC GUARANTY


          THE UNDERSIGNED, NEW CENTURY FINANCIAL CORPORATION, HEREBY (1) AGREES
THAT EACH REFERENCE TO THE CREDIT AGREEMENT, OR WORDS OF SIMILAR IMPORT,
CONTAINED IN THE THIRD AMENDED AND RESTATED GUARANTY DATED AS OF MAY 29, 1998
(THE "GUARANTY") BY THE UNDERSIGNED TO THE LENDERS AND THE AGENT, SHALL BE A
REFERENCE TO THE CREDIT AGREEMENT AS AMENDED BY THE FOREGOING AMENDMENT, (2)
CONFIRMS THAT THE GUARANTY SHALL REMAIN IN FULL FORCE AND EFFECT AFTER GIVING
EFFECT TO THE FOREGOING AMENDMENT, AND (3) CONFIRMS AND ACKNOWLEDGES THAT ITS
REPRESENTATIONS AND WARRANTIES SET FORTH IN SECTION 15 OF THE GUARANTY ARE TRUE
AND CORRECT AS OF THE DATE OF THE FOREGOING AMENDMENT.

                              NEW CENTURY FINANCIAL CORPORATION

                              By  /s/ Brad A. Morrice
                                 ----------------------------------------------
                                 Its  Vice Chairman and President
                                    -------------------------------------------
<PAGE>

                         REAFFIRMATION OF NCCC GUARANTY


          THE UNDERSIGNED, NC CAPITAL CORPORATION, HEREBY (1) AGREES THAT EACH
REFERENCE TO THE CREDIT AGREEMENT, OR WORDS OF SIMILAR IMPORT, CONTAINED IN THE
THIRD AMENDED AND RESTATED GUARANTY DATED AS OF DECEMBER 11, 1998 (THE
"GUARANTY") BY THE UNDERSIGNED TO THE LENDERS AND THE AGENT, SHALL BE A
REFERENCE TO THE CREDIT AGREEMENT AS AMENDED BY THE FOREGOING AMENDMENT, (2)
CONFIRMS THAT THE GUARANTY SHALL REMAIN IN FULL FORCE AND EFFECT AFTER GIVING
EFFECT TO THE FOREGOING AMENDMENT, AND (3) CONFIRMS AND ACKNOWLEDGES THAT ITS
REPRESENTATIONS AND WARRANTIES SET FORTH IN SECTION 15 OF THE GUARANTY ARE TRUE
AND CORRECT AS OF THE DATE OF THE FOREGOING AMENDMENT.

                              NC CAPITAL CORPORATION


                              By  /s/ Brad A. Morrice
                                 -------------------------------------------
                                 Its   Chairman & CEO
                                     ---------------------------------------