
<PAGE>   1
                                                                     EXHIBIT 5.1



                 [AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.]


                                 June 18, 1999


Century Business Services, Inc.
6480 Rockside Woods Boulevard, South
Suite 330
Cleveland, Ohio  44131

Ladies and Gentlemen:

         We have acted as counsel to Century Business Services, Inc., a
Delaware corporation (the "COMPANY"), in connection with the filing of a
registration statement on Form S-4 (as amended, the "REGISTRATION STATEMENT")
with the Securities and Exchange Commission pursuant to the Securities Act of
1933, as amended (the "SECURITIES ACT"), and the rules and regulations
promulgated thereunder.  The Registration Statement relates to an aggregate of
up to 15,000,000 shares (the "SHARES") of common stock, par value $0.01 per
share ("COMMON STOCK"), of the Company, which may be issued by the Company from
time to time in the future on the completion of acquisitions of assets,
businesses or securities ("ACQUISITIONS") or the conversion of or payment of
interest on convertible securities issued by the Company in connection with
such acquisitions of other assets, businesses or securities.

         We have, as counsel, examined originals or copies, certified or
otherwise identified to our satisfaction, of such corporate records,
agreements, documents and other instruments, and such certificates or
comparable documents of public officials and of officers and representatives of
the Company, and have made such inquiries of such officers and representatives,
and reviewed such questions of law as we have deemed relevant, necessary or
appropriate as a basis for the opinion hereinafter set forth.

         In such examination, we have assumed the genuineness of all
signatures, the authenticity of all documents submitted to us as originals, the
conformity to original documents of documents submitted to us as certified or
photostatic copies and the authenticity of the originals of such latter
documents.  As to all questions of fact material to this opinion, we have
relied upon representations of the Company.

         Based upon such examination and representations, we advise you that,
in our opinion, when the Registration Statement becomes effective under the
Securities Act, and the Shares are
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AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.

Century Business Services, Inc.
June 18, 1999
Page 2


issued in connection with the Acquisitions, the Shares will be validly issued,
fully paid and non-assessable.

         In connection with the opinion expressed above, we have assumed that,
at or prior to the time of the delivery of any Shares, (i) the Board of
Directors of the Company (and, to the extent required by applicable law, the
stockholders of the Company) shall have duly authorized and approved the
Acquisition pursuant to which such Shares are being issued and shall have duly
authorized and approved the issuance of such Shares pursuant to such
Acquisition and such authorizations shall not have been modified or rescinded,
(ii) there are sufficient authorized and unissued shares of Common Stock to
satisfy such issuance, (iii) the Registration Statement shall have been
declared effective and such effectiveness shall not have been terminated or
rescinded, (iv) there shall not have occurred any change in law affecting the
validity or enforceability of such Shares and (v) there shall not have occurred
any change in the Certificate of Incorporation or Bylaws of the Company.  We
have also assumed that the consideration received by the Company for such
Shares in any such Acquisition is lawful consideration and equals or exceeds in
value the aggregate par value of the Shares issued and that neither the
issuance or delivery of such Shares, nor the compliance by the Company with the
terms of such Shares, will violate any applicable law or will result in a
violation of any provision of any instrument or agreement then binding upon the
Company, or any restriction imposed by any court or governmental body having
jurisdiction over the Company.

         The foregoing opinion is limited to the corporate laws of the State of
Delaware and the Federal laws of the United States of America, and we express
no opinion as to the effect on the matters covered by this letter of any other
law.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.  In addition, we consent to the reference to us under
the caption "Legal Matters" in the prospectus forming a part of the
Registration Statement.

         This opinion is rendered solely to you in connection with the above
matter.  This opinion may not be relied upon by you for any other purpose.

                                  Sincerely yours,



                                  /S/ AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.
                                  AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.

