The appointment marks the agency's first step towards reshaping its management line-up after the departure of some of its senior figures last year. Jonathan Rigby, FCB's former managing director, left in May.
McCarthy joins from Nitro after less than a year in the job. At Nitro he ran the Nike, Coca-Cola and Credit Suisse accounts.
Before that, he was the global business director on the HSBC and Kellogg accounts at JWT. He has also had stints at Euro RSCG and CDP, where he worked as an account handler. He began his advertising career at Publicis as a graduate trainee.
McCarthy will start at FCB in February, reporting to the agency's recently appointed chief executive, Nigel Jones.
Jones commented: "McCarthy's infectious passion and energy mean he will be a fantastic driver of change."
McCarthy said: "The whole industry is changing fast, and FCB will be right at the forefront of this change. There is no better place to be right now."
McCarthy's appointment is the first of several senior hirings at FCB as Jones looks to bring expertise from other disciplines to create a broader communications agency.
Jones is also looking for a creative director, following the departure of Al Young at the end of last year, and a planning director, after David Bain left the agency in November 2004. Bain, Young and Rigby had been hired by the FCB chairman, John Banks, as part of a "dream team". Banks retired from FCB in June.
Over the past two years, FCB has lost significant pieces of domestic business, including the Waitrose, Weetabix, Burton's Foods and Hilton Hotels accounts.
The agency has failed to replace the lost business, with its 2004 billings falling 23 per cent to £67.6 million, according to Nielsen Media Research figures.
FCB London's major accounts include Associated Newspapers' Daily Mail, Chevrolet, Beiersdorf UK and SC Johnson.