The agency picked up the brief after a final shoot-out against Sir Martin Sorrell's WPP Group.
Chime Communications, which handled the account for the handover from Beijing to London, was eliminated at an earlier stage.
McCann Erickson, supported by other Interpublic agencies, will be responsible for all advertising, promotion, public relations and digital communications in the run-up to the 2012 London Olympic Games.
The agency will pay around £10 million in services-in-kind for the privilege of marketing the event, in return for obtaining third-tier sponsorship rights.
Chris Townsend, London 2012's commercial director, and Amanda Jennings, the head of brand and marketing for LOCOG, led the pitch.
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