He has left the company and is reported to have been replaced on an interim basis by Alan Batey, head of US sales and service, while the company searches for a replacement.
The reasons for his departure are not clear, but several reports quote a GM spokesman as saying Ewanick "failed to meet the expectations that a company has of an employee".
One report, from Reuters, quotes an anonymous source as claiming that Ewanick failed to report the financials of a five-year sponsorship deal between its Chevrolet brand and Manchester United, announced in May this year. Reuters claims Ewanick declined to comment on the matter.
Ewanick joined the US car-maker less than two years ago, rising to CMO in December 2010.
He was tasked with leading an overhaul of its marketing strategy and has made a series of controversial marketing moves, including severing its advertising ties with Facebook.
GM chose to sponsor the football club as part of its strategy to build the Chevrolet brand in Europe. In a recent interview with Marketing, Ewanick said the brand owner was to build a carpark next to the Old Trafford ground to ‘thank’ the fans ‘for letting us be a part of it’.
In a surprise move last week, GM signed a four-year deal for its Chevrolet brand to become the automotive partner Manchester United’s rival club Liverpool.
Ewanick attracted controversy when GM pulled its $10m paid advertising spend from Facebook, in the week leading up to the social networking company’s initial public offering, over scepticism about the digital ads’ ROI.
The two companies are said to be in talks again about rekindling their advertising relationship.
Meanwhile GM has also said that it would not be advertising during next year’s Super Bowl.
General Motors’ main brand in the UK is Chevrolet, which rolled out an ad campaign this weekend as part of its Olympics sponsorship. While the company also owns Vauxhall, its marketing is run out of Germany, where the car marque is called Opel.
Ewanick was also responsible for appointing Carat to its $3bn global media planning account at the start of this year.