The WPP media network saw off competition from Interpublic Group's Initiative Media and the Aegis-owned Carat.
The review was launched in December following the departure of Steve Lazano from MindShare to head US operations for Havas' MPG. He was originally poached from Mediaedge:cia to lead the Amex account.
The assessment excluded the UK market, Japan and Italy, where the media is handled by local offices of Carat, Dentsu and Publicis Groupe's ZenithOptimedia, respectively.
A spokesman for the financial services giant in the US said the review was held to ensure that the company stayed at the forefront of changes in the global media landscape and had the best resources to work on its business.