The agency will take over the account from Chime Communications, which held a one-year contract for London 2012's marketing in the run up to the handover ceremony at the Beijing Olympics.
It is understood that Sir Martin Sorrell's WPP lost out to McCann Erickson because WPP was unwilling to split the tender for marketing from the tender for market research, which is being awarded separately.
McCann Erickson will not be paid a fee for its work, as bidders were invited to say how much they were prepared to pay for the right to market the Games, which is thought to have deterred other major agencies from pitching.
McCann will pay around £10m (US$14.7 million) in service-in-kind in return for which it becomes an official sponsor of the Games. WPP's in-kind offer is understood to have fallen well short of this figure.
McCann Erickson said that the bid to promote the Games was still ongoing. WPP declined to comment.
A spokeswoman from Locog said the organisation does not comment on sponsorship deals and an announcement will be made when the deal has been completed.