141 triumphed after a two-way pitch against an undisclosed agency and has been briefed to develop a programme to promote its sponsorship of Manchester United around the world.
The agency is expected to use events, promotions and direct response campaigns to help use the sponsorship to build the brand globally.
AIG took the world by surprise when it signed a £56.4 million, four-year deal to sponsor Manchester United in April. The deal replaced the club's contract with Vodafone, which expired at the end of last season.
AIG predominantly operates in the business-to-business arena, although it does have some consumer-facing operations.
The insurance giant was also pivotal in backing Malcolm Glazer's takeover of Manchester United last summer.