The Boston-based independent has handled the motor company's Hummer account since 2000, and in March this year was awarded a small chunk of the Cadillac business without a pitch.
The move follows the arrival of Liz Vanzura as the marketing director of Cadillac. She moved from the same post at Hummer.
Burnett will keep the national and regional dealer account for GM's Pontiac brand, plus the car-maker's service and parts business.
Jim Moore, the president of the Detroit office, said the agency had not decided on the extent of the anticipated redundancies.