Miller is understood to have asked Mother to submit ideas for a new global campaign earlier this year, but later asked Leo Burnett to put forward its own work.
Leo Burnett already had a connection with Miller through Arc, which won the brief to handle Miller Genuine Draft's integrated account in November last year.
Arc was hired by the parent company SABMiller to work with Mother and handle the international roll-out of the planned new global campaign. Leo Burnett and Arc will now handle conception and roll-out for the campaign.
Andy Routley, the international brand director at SABMiller, said: "I'd like to thank Mother for its contribution to our business."
Mother won the business in February 2005 after pitching against VCCP and The Hive, the Toronto-based incumbent.