WPP last year registered the largest revenue for a global publicly listed marketing services company, pushing Omnicom out of the top spot.
The UK-based group led by chief executive Sir Martin Sorrell (pictured) was helped by its acquisition of top tier market research company TNS in October 2008.
The addition led to a 13% increase in WPP's calendar year revenue – from $10.7bn in 2008 to $12.1bn in 2009.
US-based Omnicom suffered a 12% drop in revenue from $13.4bn in 2008 to $11.7bn in 2009 as, along with the rest of the industry, it was hit by reduced client spend.
However, Omnicom was the most profitable company in the rankings, which were compiled by Marketing Services Financial Intelligence.
Despite Omnicom's post-tax profit falling 21% and WPP's holding steady, the former weighed in with a figure of $793m to the latter's $666m.
Publicis Groupe claimed third place from US-based Interpublic in revenue terms, with just a 4% drop to $6.4bn compared to Interpublic's 13% drop to $6bn.
Japan's Dentsu, France's Havas and the UK's Aegis occupied the next three places.
Dentsu was strong on profitability, with post-tax profits of $341m outstripping the combined profits of Havas, Interpublic and Aegis.
It placed fourth in the ranking of post-tax profits after Publicis Groupe, which earned $573m.