The chocolate brand, which is now owned by Mondelez, a new snacks business separated from Kraft Foods last month, began approaching agencies directly in September.
Most-recent major TV work for Toblerone, created by Ogilvy & Mather in March 2011, showed a woman devising new and interesting ways to eat a bar by breaking up its triangular segments.
The campaign included a website that asked: "What makes your Toblerone, Toblerone?" People could then upload videos and participate in monthly challenges.
It is the second win for Mcgarrybowen since it changed its name from Dentsu London earlier this year. In July, the agency picked up the £25 million Honda pan-European account.