Thomas will be based in Singapore and will report to Andrew Robertson, BBDO's worldwide president, and Michael Birkin, the chairman and chief executive of Omnicom Asia.
Thomas will not be replaced directly at Proximity. Amanda Phillips, Proximity's director of client services, will become its managing director. Cilla Snowball, the Abbott Mead Vickers BBDO chairman, will add the chairmanship of Proximity to her duties.
Phillips was one of Thomas' first hirings as he rebuilt the agency after the departures of three of its founders - Simon Hall, Chris Barraclough and Shona Forster. Phillips is the former managing director of MRM Worldwide and the IPA DM futures group chairman.
Thomas will begin working in Asia on a part-time basis from January.
Michael Baulk, the AMV Group chairman and chief executive, said: "Appointing Chris to run Proximity was one of the best decisions of my career. Even I couldn't have predicted what a fantastic job he has done."
Omnicom's weakness in Asia is said to have been a key reason it lost out to WPP in the 2004 HSBC pitch. Thomas led Omnicom's pitch.
Comment, page 44.