At the same time, the financial services company is also shifting its £11 million media account in Italy to MindShare from ZenithOptimedia.
The accounts were moved without a competitive pitch in a bid to achieve efficiencies across the globe.
WPP's Ogilvy & Mather has been handling the Amex creative account on a global basis since 1962.
MindShare had been in talks with the company for a number of months, and it made a pitch for the business several weeks ago.
The agency now handles American Express' media business in all major markets except Japan. The Japanese market is handled by the Tokyo-based advertising agency Dentsu.
The UK account was last moved 15 years ago, when Carat picked up the business following a competitive pitch against the media department of Ogilvy & Mather.
According to Nielsen Media Research, American Express spent £4.6 million on press in the past 12 months. The company's next biggest investment in media was television, with a total of £3.8 million. This was followed by a spend of £1.8 million on digital.
MindShare and American Express declined to comment.