The WPP agency was awarded the business after pitching against incumbent OMD and Starcom MediaVest in a three-month review process led by Trace Rutland, commercial manager of media innovation for Mars North America.
The agency assumes responsibility for all of the confectionery giant's media planning and buying, and strategic side of the business, with immediate effect.
It will be managed by teams working from MediaCom's offices in Toronto and Montreal. The review follows the $23bn merger of the global confectionery giants Mars and The Wrigley Company in October 2008.
Jamie Edwards, chief executive of MediaCom Canada, promised a "fresh communications approach to help fuel brand growth".
The move followed the appointment of Publicis Groupe's MediaVest in the US at the start of the summer, to handle the new $500m Mars/Wrigley media account.
It marks the second major new business win for MediaCom Canada this month, after winning the media business for Yum! Restaurants – owner of KFC, Pizza Hut and Taco Bell brands in Canada – last week.