The deal, believed to have included a cash down-payment and payments for the agency's senior staff over the next five years, will be completed through WPP's wholly owned global branding and identity unit, The Brand Union.
MCA, which has a client list including British Airways, Cadbury Schweppes, Sony and NTL, will later be merged with WPP's branding consultancy Banner McBride.
MCA has offices in London, New York and Stockholm. It recorded revenues of £8.8m in 2000 and net assets of £1.9m.
WPP's share price fell 6.5p this morning to 657p in London.