The Greenlight appointment is expected to kick-start growth in American Express' UK search spend over the next 12 months, with the figure tipped to pass the seven-figure mark. The firm spent almost £2m on digital marketing in 2006, according to Nielsen Media Research.
Greenlight beat shortlisted agencies Media Contacts and The Search Works in a competitive pitch process, after Amex moved its search account out of incumbent Online Media Group.
The three shortlisted agencies were picked from a longlist that included many of the biggest search agencies in the UK.
Most of American Express' advertising account is held by WPP agencies Ogilvy and MindShare, but the US-based firm has turned to independent agencies to handle its digital media spend.
American Express moved its £11m UK media planning and buying account from Aegis-owned Carat to MindShare in November last year after 15 years with the agency. MindShare now holds the American Express account around the world apart from in Japan, where the credit card firm prefers Tokyo-based agency Dentsu.
Digital media agency TBG London runs American Express' online media planning and buying and direct response work in the UK, and now Greenlight has joined the firm's roster.
Greenlight's victory is the latest sign that established names in search are facing increased competition from independent agencies keen to gain a slice of paid-for search spend.
In August, Telegraph Media Group moved its search account out of media agency Universal McCann to independent search agency Propellernet.
The paid-search sector grew by 44% to £762m in the first half of 2007 and now accounts for 57.1% of online ad spend, according to figures released by the IAB and PricewaterhouseCoopers.
Greenlight adds American Express to a client list that includes HMV, Monarch Airlines, Hotels.com and online automotive marketplace Autobytel.co.uk.