Small Business Saturday, which is to take place on 6 December, originated in the US and was brought over to the UK last year.
It is believed there was a competitive pitch for the work but it is not clear which agencies The Academy pitched against.
It joins the American Express UK roster, which includes MHP, Ogilvy, Consolidated, Talk PR and Eulogy.
The appointment was confirmed by an American Express spokeswoman. The Academy declined to comment.