Sinclair's most recent role was as the creative director at the US independent agency See:, which serviced the $150 million account for Electronic Arts (EA Games). However, the games manufacturer moved the account to Wieden & Kennedy in January.
Sinclair, 40, and his former partner Ed Morris, now at Lowe, used to work at DDB London, where they created "lost dog" for Volkswagen. The team also collaborated on Levi's and One2One at Bartle Bogle Hegarty.
Their most famous spot was "double life" for Sony PlayStation, produced at TBWA\London.
The pair split in 2003, when Sinclair moved to Ogilvy & Mather to head the creative output on its Ford account.
He said: "JWT is a fabulous place to be. With Craig Davis (the worldwide chief creative officer) and Michael Maedel (the worldwide president), it's like being invited on to the bridge of the Enterprise."
Morris and Sinclair began their careers at Miller Leaves in 1991 and remained there throughout mergers with Bainsfair Sharkey Trott and TBWA GGT Simons Palmer.
Sinclair's hiring follows that of Axel Chaldecott, one of the founding partners of HHCL, who joined JWT last month to work on HSBC.
Davis said: "James is perfect for the new JWT."