The moves sees 32-year-old James - who during his time at MGM was the youngest head of a London agency - return to the Paris Gardens offices he vacated in 1997.
His appointment completes a wholesale change of the agency's management line-up - four months ago Rob Norman was hired as the Mediaedge:cia chairman to replace the outgoing chief executive, Fiona McAnena.
The appointments mean that Mediaedge:cia is now run by people from traditional media backgrounds. Previous incumbents have been drawn from the ranks of clients, creative agencies and brand consultancies. Commenting on James' appointment, Norman said: "He has the energy and skills we require to lead the company to the next stage of its development."
As part of the agency's reorganisation, Norman has created two new client services director positions to be filled by Ian Redman and Andy Martin.
Martin will work alongside Tim Davis, the head of direct services, and with Wunderman.
This follows last month's announcement that Norman had formed a strategic partnership between Media-edge:cia and its sister WPP agency Wunderman to offer joint planning and execution of campaigns.
Whitmore, a former managing director of The Media Edge, has left the agency without a job to go to.