Mr Niccol, who previously headed the Mexican food chain Chipotle, was brought into Starbucks to help turn the business around.
But he faced criticism over his plan to commute almost 1,000 miles (1,600km) from his family home in Newport Beach, California, to the firm’s headquarters in Seattle on a corporate jet.
Critics saw it as in contradiction with the company’s public stance on green issues.
Starbucks is due to release its full results next week. It shares dropped 4% on Tuesday as it suspended its financial forecasts for the next year due to “current state of the business”.
Starbucks’ former chief executive Laxman Narasimhan, who was ousted after a year-and-a-half in the role, had attempted to revitalised the chain’s menu.
He said in summer he wanted to add new items such as boba drinks and an egg sandwich with pesto as well speedier service in stores.
However, weeks later he left.
Starbucks has also been grappling with protests and boycott campaigns on social media tied to the Israel-Gaza war and a union fight in the US.
A union working to organise baristas in the US posted a message on social media expressing “solidarity” with Palestine, shortly after the 7 October attack by Hamas.
The post, which the union said was not authorised by leaders, spread rapidly despite being taken down, and sparked backlash against the coffee giant.
Starbucks said it disagreed with the union’s statement. It described its official position as condemning “violence in the region”.
