The appointment comes ahead of a major product offensive between now and 2010. GM is planning to launch 45 models in Europe over the next five years, boosting the ranges of its Vauxhall, Chevrolet and Opal marques.
Universal McCann will take responsibility for media in all of its European markets with the exception of the UK - 44 in total.
It takes over the important German account from Carat, which also pitched against Initiative for the UK business. GM owns the Daewoo - to be rebranded Chevrolet on 1 January - Saab and Vauxhall marques in this country .
GM Europe vice-president of sales, marketing and aftersales Jonathan Browning said that the consolidation would offer significant economies of scale.
The retention of GM in the UK is a major boost for Initiative after it lost the £206m Unilever UK media account to MindShare last month, and secures the agency's immediate future as a separate operation in the UK.
Yet it is likely to have paid a price in reduction of agency fees.
It is the second time in little over a year that Initiative has had to defend the business. Last autumn it retained the Vauxhall and Daewoo accounts and picked up Saab's £20m business from Universal McCann.