"I was part of the inception of GM Planworks when we first won the planning assignment in 2000, and I am very proud of the successes and innovations we have achieved with the support of the GM client," Mr. Donlin said in a statement. "It is simply the right time for me to take stock and consider my next career move, and I want to do that while spending more quality time with my kids, the youngest of which graduates from high school next year."
Mr. Donlin has worked in media since 1979, when he joined D'Arcy Masisus Benton & Bowles.
In a statement, SMG CEO Laura Desmond said a transition plan was being finalized and a successor would likely be announced next week.
"Dennis has been an integral part of our Planworks operation and a big part of the success that division has achieved," said Ms. Desmond. "We wish him the very best in his future endeavors."