The contract, which was previously managed in-house, relates to the sites on the Bury to Altrincham and Eccles lines until 2011, and will include management of all the trams' interior sites, window advertising and exteriors, as well as some 4-sheet poster sites at tram stops.
JCDecaux currently runs a separate contract to manage sites at tram stop platforms.
The win is an extension of CBS's portfolio of tram operators, which includes Sheffield Supertram, Croydon Tram and Midland Metro.
In Manchester, CBS manages the advertising on the city's buses, as well as sites at railway stations through its Northern Rail and First Group franchises.
CBS also manages advertising on all local and national buses within Stagecoach UK Bus Group.
The Metrolink is used by an estimated 55,000 passengers a day and 20.1 million a year.
Andy Morris, Stagecoach's managing director of Light Rail at Metrolink, said it was CBS's "experience and knowledge of the transport market" that he hoped would help the company "continue to grow and develop the advertising infrastructure"