The review was initiated late last year.
The decision was made due to MindShare's combination of resources in multiple markets around the world, said Judy Tenzer, an American Express spokeswoman.
Alec Gerster, Initiative's CEO, said, "We took them to the wire. We really made these guys sweat bullets."
The spokeswoman said today's decision does not affect the U.K. market, Japan or Italy, where the media is handled by local offices of Carat, Dentsu and Publicis Groupe's Zenith Optimedia, respectively.
American Express spent $357 million in measured media in 2003, according to TNS Media Intelligence/CMR.
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Mercedes Cardona contributed to this report.