Thomas, 41, will take over as the chief executive in September when Simon Hall, the current chief executive, moves to the role of chairman in order to focus on his duties as the chief executive of Proximity Worldwide.
Thomas' role at Proximity reunites him with Abbott Mead Vickers BBDO's group chief executive, Michael Baulk. The pair previously worked together during Thomas's ten-year spell at AMV, before his emergence as a central figure in the merger of Lintas and Lowe.
Thomas was forced to quit Lowe in the spring following a difficult year for the UK shop, which suffered a number of account loses including the £43 million Orange business.
The reunion of Baulk and Thomas is expected to improve further the successful working relationship between Proximity and its above-the-line affiliate AMV.
Proximity, AMV and Wolff Olins won the Royal Mail's integrated communications account earlier this year following a joint pitch. AMV and Proximity were also successful in their joint pitch for Norwich Union Direct at the end of last year.
Baulk said: "Chris is an impressive addition to our group's stable of talent. His width of experience and his track record of supporting great work makes him the ideal chief executive to take Proximity forward."
Thomas added: "Proximity London is one of the stars of the AMV Group. Clients are increasingly looking for a more varied and flexible mix of communications tools to reach today's consumer.
"In order to grow, agencies must think far beyond pure advertising. My brief is to make sure that Proximity achieves a significant share of this growth."
Proximity is the fourth-largest DM business in the UK, with clients including Volkswagen and Sainsbury's.
Thomas replaced Paul Hammersley as the Lowe chief executive in 2001, when Hammersley left to run Lowe's New York operation.
Although Thomas succeeded in bedding down the Unilever account, the loss last year of the bulk of the Vauxhaul account, coupled with the loss of Weetabix and Orange, led Lowe's international management to rethink its London line-up.
Tim Lindsay, the network's European chief, appointed Matthew Bull as chief executive to replace Thomas. Bull took up his new role this week.