McCarter joins the agency this month and will be responsible for overseeing the strategy across Ogilvy's key global accounts, with a primary focus on the Unilever business.
He will also spearhead strategic development and initiatives across the network, reporting directly to Colin Mitchell, Ogilvy's worldwide head of planning.
Ogilvy has been without a planning chief in the region since Russell Davies left the agency in June last year to join R/GA London.
McCarter will begin his second spell at Ogilvy, having previously worked as a planning director in the agency's London office between 1992 and 1997.
After that, McCarter moved to Bartle Bogle Hegarty, before launching a freelance planning practice in New York, where he worked directly with clients including Diageo, Unilever, Philips and Absolut.
In this role, McCarter also acted as a consultant to a number of advertising agencies, including Ogilvy; and, last year, he helped the network win the global SC Johnson and Philips ad accounts.
Mitchell said: "Brian is a world-class planner. Everyone who works with him praises his subtle insight and ability to get to the very heart of brands."