GM is expected to announce Ewanick's departure later today, a person familiar with the situation told Auto News.
A GM spokesman didn't immediately return a call seeking comment.
Mr. Ewanick's departure spotlights the volatility within GM's executive ranks under Dan Akerson, who became GM's CEO a few months after Mr. Ewanick joined.
Since then, Akerson has installed a new product development chief and replaced the head of GM's troubled European operations several times.
Last year, former Chief Financial Officer Chris Liddell, a high-profile recruit from Microsoft Corp., left the company and was replaced by current CFO Dan Amman. GM also has replaced its heads of manufacturing and engineering as well as its OnStar unit during Mr. Akerson's two-year tenure.
Mr. Ewanick recently led a massive consolidation of the external marketing and advertising agencies that work with GM. Last spring, he moved the account for Chevrolet's creative work from dozens of agencies globally to just one firm, Commonwealth of Detroit.
He also put all of GM's media buying duties under London-based Aegis' Carat unit, ending its work with dozens of smaller agencies. Combined, those moves are expected to save GM $2 billion over five years, the company has said.