The move marks a near-global consolidation for the financial services giant, which is also swapping its media account in Italy from Publicis-owned ZenithOptimedia to MindShare.
It is understood the decision to change agencies in the UK and Italy was made without a formal competitive pitch.
WPP-owned MindShare now holds the credit card company's business across all its territories, except in Japan where the media planning and buying account will continue to be handled by Tokyo-based advertising agency Dentsu.
Carat has handled the AmEx UK account for the past 15 years. American Express' global creative network remains WPP-owned Ogilvy and Mather.
Neither MindShare UK nor Carat UK were available for comment.
According to Nielsen Media Research, the total UK media spend, including direct mail for the last 12 months was £25.2m. Without direct mail it totalled £11.3m.
According to Nielsen figures for the last 12 months, Amercian Express spent £1.8m on online, £0.6m on outdoor, £4.6m on press, £0.5 on radio and £3.8m on television advertising.
American Express launched its RED card in March this year, which allows consumers to give 1% of the total spend on their RED cards to the Global Fund to fight AIDS in Africa.