The global marketing officer for America's biggest car-maker has resigned amid a shake-up after it controversially pulled advertising from Facebook.
GM said that Mr Ewanick decided to step down with immediate effect.
He joined the firm in 2010 and faced the challenge of creating a new image for GM, following a massive bailout of $49.5bn (£31.5bn) by the US government in 2009.
He was in charge of marketing when the company decided to pull its $10m (£6m) Facebook advertising contract in May.
The move surprised many in the business and came just weeks before Facebook launched its much-hyped flotation.
Industry experts had voiced concern over how Facebook could properly monetise social media if major corporations decided to withdraw like GM.
At the time of Facebook's flotation, Pivotal Research Group internet and media analyst Brian Wieser said GM's decision raised important issues about Facebook as a business model.
Mr Wieser said: "It looks likely that it will be one of the most important ad-supported media properties but it's not certain because there will be marketers who are challenged to prove the effectiveness of the marketing vehicle."
The car firm has also opted out of advertising during the 2013 Super Bowl, a huge event for marketing, which sees many manufacturers scramble for prime commercial air time.
According to reports, vice-president of US sales and service Alan Batey will temporarily replace Mr Ewanick.
The executive re-shuffle has also hit Vauxhall, the company’s European arm, as former chief executive Karl-Friedrich Stracke was ousted earlier this month.
GM, which is still 26% owned by the US government, reported a net profit of $1bn (£636m) in the first quarter.