Sinclair was hired by Ogilvy's executive creative director, Malcolm Poynton, in June last year and was tasked to improve the output of the agency's £95 million, pan-European Ford account.
In his new position, Sinclair will work alongside See's UK management team of Phil Gault and Simon Law. The agency is the London arm of the San Francisco-based shop, which was previously branded Odiorne Wilde Narraway & Partners.
In the US, the agency has recently undergone a revamp and at the beginning of last year, Harry Cocciolo arrived from Goodby Silverstein & Partners in Australia to take charge of a reinvigorated creative department.
The aim is to complete a similar reinvigoration of the London shop and the agency is set to announce the arrival of several new clients.
See's main account is the videogames distributor Electronic Arts, which it holds globally and operates out of both its London and San Francisco offices. It spends £12 million in the UK.
Sinclair joined O&M from Lowe, where he worked as a senior copywriter with his long-time partner, Ed Morris. During a stint at TBWA\London, the team made the "double life" campaign for Sony PlayStation.
Paul Smith, O&M's European creative director, said: "Working with myself and others, James has made a considerable contribution to our work on Ford. A successor will be announced following consultation with our client."
Sinclair refused to comment.