<PAGE>

                      SECURITIES AND EXCHANGE COMMISSION
                              Washington, DC  20549

                                   SCHEDULE 13G
                                  (Rule 13d-102)

             INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT
           TO RULES 13d-1(b)(c), AND (d) AND AMENDMENTS THERETO FILED
                            PURSUANT TO RULE 13d-2(b)
                               (Amendment No.   )(1)
                                             ---

                                 Mediacom Communications Corporation
-------------------------------------------------------------------------------
                                 (Name of Issuer)

                               Class A Common Stock
--------------------------------------------------------------------------------
                          (Title of Class of Securities)

                                   58446K105
--------------------------------------------------------------------------------
                                 (CUSIP Number)

                                 12/31/2000
--------------------------------------------------------------------------------
(Date of Event Which Requires Filing of this Statement)

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

     /X/   Rule 13d-1(b)

     / /   Rule 13d-1(c)

     / /   Rule 13d-1(d)


-----------------
(1)  The remainder of this cover page shall be filled out for a reporting
     person's initial filing on this form with respect to the subject class of
     securities, and for any subsequent amendment containing information which
     would alter the disclosures provided in a prior cover page.

     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 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).


<PAGE>

CUSIP No. 58446K105                   13G                   Page 2 of 8 Pages

--------------------------------------------------------------------------------
  1.  NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

         The TCW Group, Inc.
--------------------------------------------------------------------------------
  2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                    (a)  / /
                                                                    (b)  /X/
--------------------------------------------------------------------------------
  3.  SEC USE ONLY


--------------------------------------------------------------------------------
  4.  CITIZENSHIP OR PLACE OF ORGANIZATION

         Nevada corporation
--------------------------------------------------------------------------------
                 5.  SOLE VOTING POWER
  NUMBER OF          -0-
   SHARES      -----------------------------------------------------------------
BENEFICIALLY     6.  SHARED VOTING POWER
  OWNED BY           5,677,937
    EACH       -----------------------------------------------------------------
 REPORTING       7.  SOLE DISPOSITIVE POWER
  PERSON             -0-
   WITH        -----------------------------------------------------------------
                 8.  SHARED DISPOSITIVE POWER
                     5,677,937
--------------------------------------------------------------------------------
  9.  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
      5,677,937
--------------------------------------------------------------------------------
 10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*
                                                                             / /
--------------------------------------------------------------------------------
 11.  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
       9.4%(see response to Item 4)
--------------------------------------------------------------------------------
 12.  TYPE OF REPORTING PERSON*
      HC/CO
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


<PAGE>

CUSIP No. 58446K105                  13G                   Page 3 of 8 Pages

--------------------------------------------------------------------------------
  1.  NAMES OF REPORTING PERSONS
      I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

         Robert Day
--------------------------------------------------------------------------------
  2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                   (a)  / /
                                                                   (b)  /X/
--------------------------------------------------------------------------------
  3.  SEC USE ONLY


--------------------------------------------------------------------------------
  4.  CITIZENSHIP OR PLACE OF ORGANIZATION

         United States Citizen
--------------------------------------------------------------------------------
                 5.  SOLE VOTING POWER
  NUMBER OF          -0-
   SHARES      -----------------------------------------------------------------
BENEFICIALLY     6.  SHARED VOTING POWER
  OWNED BY           5,677,937
    EACH       -----------------------------------------------------------------
 REPORTING       7.  SOLE DISPOSITIVE POWER
  PERSON             -0-
   WITH        -----------------------------------------------------------------
                 8.  SHARED DISPOSITIVE POWER
                     5,677,937
--------------------------------------------------------------------------------
  9.  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
      5,677,937
--------------------------------------------------------------------------------
 10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*
                                                                             / /
--------------------------------------------------------------------------------
 11.  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
       9.4% (see response to Item 4)
--------------------------------------------------------------------------------
 12.  TYPE OF REPORTING PERSON*
      HC/IN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


<PAGE>

                                                             Page 4 of 8 Pages

Item 1(a).  Name of Issuer:

            Mediacom Communications Corporation

Item 1(b).  Address of Issuer's Principal Executive Offices:

            100 Crystal Run Road
            Middletown, NY 10941

Item 2(a).  Name of Persons Filing:
Item 2(b).  Address of Principal Business Office, or if None, Residence:
Item 2(c).  Citizenship:

            The TCW Group, Inc.
            865 South Figueroa Street
            Los Angeles, CA 90017
            (Nevada Corporation)

            Robert Day
            865 South Figueroa Street
            Los Angeles, CA 90017
            (United States Citizen)

Item 2(d).  Title of Class of Securities:

            Class A Common Stock

Item 2(e).  CUSIP Number:

            58446K105


<PAGE>

                                                               Page 5 of 8 Pages

Item 3.     If This Statement Is Filed Pursuant to Sections 240.13d--1(b), or
            240.13d--2(b) or (c), Check Whether the Person Filing is a:

            (a)  / /  Broker or dealer registered under Section 15 of the
                      Exchange Act (15 U.S.C. 78o).
            (b)  / /  Bank as defined in Section 3(a)(6) of the Act
                      (15 U.S.C. 78c).
            (c)  / /  Insurance company as defined in Section 3(a)(19) of the
                      Exchange Act (15 U.S.C. 78c).
            (d)  / /  Investment company registered under Section 8 of the
                      Investment Company Act of 1940 (15 U.S.C. 80a-8).
            (e)  / /  An investment adviser in accordance with
                      Section 240.13d-1(b)(1)(ii)(E).
            (f)  / /  An employee benefit plan or endowment fund in accordance
                      with Section 240.13d-1(b)(1)(ii)(F).
            (g)  /X/  A parent holding company or control person in accordance
                      with Section 240.13d--1(b)(1)(ii)(G).

                                (see Item 7)
                                 The TCW Group, Inc.
                                 Robert Day (individual who may be deemed to
                                 control The TCW Group, Inc. and other entities
                                 which hold the Class A Common Stock of the
issuer)

            (h)  / /  A savings association as defined in Section 3(b) of the
                      Federal Deposit Insurance Act (12 U.S.C. 1813);
            (i)  / /  A church plan that is excluded from the definition of an
                      investment company under Section 3(c)(14) of the
                      Investment Company Act of 1940 (15 U.S.C. 80a-3);
            (j)  / /  Group, in accordance with Section 240.13d--1(b)(1)(ii)(J).
      
      If this statement is filed pursuant to Rule 13d-1(c), check this box. / /


<PAGE>

                                                              Page 6 of 8 Pages

Item 4.   Ownership **

          THE TCW GROUP, INC.
          (a)  Amount beneficially owned: 5,677,937()
          (b)  Percent of class:  9.4%
          (c)  Number of shares as to which such person has:
               (i)   Sole power to vote or to direct the vote:
                     none.
               (ii)  Shared power to vote or to direct the vote:
                     5,677,937
               (iii) Sole power to dispose or to direct the disposition of:
                     none.
               (iv)  Shared power to dispose or to direct the disposition of:
                     5,677,937

          ROBERT DAY***
          (a)  Amount beneficially owned: 5,677,937()
          (b)  Percent of class:  9.4%
          (c)  Number of shares as to which such person has:
               (i)   Sole power to vote or to direct the vote:
                     none.
               (ii)  Shared power to vote or to direct the vote:
                     5,677,937
               (iii) Sole power to dispose or direct the disposition of:
                     none.
               (iv)  Shared power to dispose or to direct the disposition of:
                     5,677,937

------------------
(**)  The filing of this Schedule 13G shall not be construed as an admission
      that the reporting person or any of its affiliates is, for the purposes of
      Section 13(d) or 13(g) of the Securities Exchange Act of 1934, the
      beneficial owner of any securities covered by this Schedule 13G.  In
      addition, the filing of this Schedule 13G shall not be construed as an
      admission that the reporting person or any of its affiliates is the
      beneficial owner of any securities covered by this Schedule 13G for any
      other purposes than Section 13(d) of the Securities Exchange Act of 1934.

(***) Shares reported for Robert Day include shares reported for The TCW Group,
      Inc. 


<PAGE>

                                                               Page 7 of 8 Pages

Item 5.     Ownership of Five 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 five percent of the class of securities, check
            the following / /.

Item 6.     Ownership of More than Five Percent on Behalf of Another Person.

            Various persons other than as described in Item 4 have the right to
receive or the power to direct the receipt of dividends from, or the proceeds
from the sale of, the Class A Common Stock of Mediacom Communications
Corporation.

Item 7.     Identification and Classification of the Subsidiary Which Acquired
            the Security Being Reported on by the Parent Holding Company.

            SEE Exhibit A.

Item 8.     Identification and Classification of Members of the Group.

            Not applicable.  SEE Exhibits A and B.

Item 9.     Notice of Dissolution of Group.

            Not applicable.

Item 10.    Certification.

            Because this statement is filed pursuant to Rule 13d--1(c), the
            following certification is included:

            By signing below I certify that, to the best of my knowledge and
            belief, the securities referred to above were acquired in the
            ordinary course of business and were not acquired and are not held
            for the purpose of or with the effect of changing or influencing the
            control of the issuer of the securities and were not acquired and
            are not held in connection with or as a participant in any
            transaction having that purpose or effect.


<PAGE>

                                                               Page 8 of 8 Pages

                                  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.


Dated this 12th day of February, 2001.


                                        The TCW Group, Inc.

                                    By: /s/ Linda D. Barker
                                        ------------------------------
                                        Linda D. Barker
                                        Authorized Signatory


                                        Robert Day

                                    By: /s/ Linda D. Barker
                                        ------------------------------
                                        Linda D. Barker
                                        Under Power of Attorney dated
                                        November 6, 2000, on File with
                                        Schedule 13G for Retek, Inc.
                                        dated November 8, 2000.








<PAGE>

                                   EXHIBIT A

                 RELEVANT SUBSIDIARIES OF PARENT HOLDING COMPANY

PART A: TCW ENTITIES

PARENT HOLDING COMPANY:

       The TCW Group, Inc.
      
       Robert Day (an individual who may be deemed to control The TCW Group,
       Inc.)


RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13D-1(B):

(i)  Trust  Company of the West, a California corporation and a bank as  defined
     in Section 3(a)(6) of the Securities Exchange Act of 1934.

(ii) TCW  Asset  Management Company, a California corporation and an  Investment
     Adviser  registered  under Section 203 of the Investment  Advisers  Act  of
     1940.

(iii)      TCW  Investment Management Company, a California corporation  and  an
     Investment Adviser registered under Section 203 of the Investment  Advisers
     Act of 1940.

Note:     No Class A Common Stock of Mediacom Communications Corporation is held
     directly by The TCW Group, Inc. Other than the indirect holdings of The TCW
     Group, Inc., no Class A Common Stock of Mediacom Communications Corporation
     is  held  directly or indirectly by Robert Day, an individual  who  may  be
     deemed to control The TCW Group, Inc.

PART B: NON TCW ENTITIES

PARENT HOLDING COMPANY:

        Robert Day (an individual who may be deemed to control the entities
        described below which are not subsidiaries of The TCW Group, Inc.)


RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13D-1(B):

        Oakmont Corporation, a California corporation and an Investment Adviser
        registered under Section 203 of the Investment Advisers Act of 1940.





                             A-1





<PAGE>

                                   EXHIBIT B

                            JOINT FILING AGREEMENT


     The undersigned acknowledge and agree that the foregoing statement on
Schedule 13G is filed on behalf of each of the undersigned and that all
subsequent amendments to this statement on Schedule 13G shall be filed on behalf
of each of the undersigned without the necessity of filing additional joint
acquisition statements. The undersigned acknowledge that each shall be
responsible for the timely filing of such amendments, and for the completeness
and accuracy of the information concerning him or it contained therein, but
shall not be responsible for the completeness and accuracy of the information
concerning the other, except to the extent that he or it knows or has reason to
believe that such information is accurate.


Dated this 12th day of February, 2001.

                                        The TCW Group, Inc.

                                    By: /s/ Linda D. Barker
                                        ------------------------------
                                        Linda D. Barker
                                        Authorized Signatory


                                        Robert Day

                                    By: /s/ Linda D. Barker
                                        ------------------------------
                                        Linda D. Barker
                                        Under Power of Attorney dated
                                        November 6, 2000, on File with
                                        Schedule 13G for Retek, Inc.
                                        dated November 8, 2000.












                                      B-1