Eugene Mullins, a longtime friend of Stroger's, said he was leaving for a job in the private sector in marketing, but refused to release details on the new position.
"A new administration is here, and so it's time to move on," he said.
Mullins became part of the investigation into former deputy chief of staff Carla Oglesby, who was charged last month with funneling county contracts to her personal company for financial gain.
Mullins was named Public Official C in the state's attorney proffer that charged Oglesby.
Mullins said his departure has nothing to do with the investigation. He said he planned to leave at the end of this year even if Stroger won reelection.