"I've made the decision to leave Microsoft," Soltero wrote. "The past 4 years have been an incredible experience. I'm humbled to have been a part of building something that 100M+ users depend on every day and grateful to have worked with some amazing people."
Soltero will leave the company by the end of the year, according to ZDNet, and will "go back to doing entrepreneurial activities." He joined Microsoft in 2014 when the company bought email startup Acompli, which he co-founded. Soltero led Outlook and Outlook Mobile, and was later head of strategy for Office. He began running Cortana in March.
Microsoft has been working to shift Cortana from a standalone assistant into an "assistance aide" in the age of Amazon's Alexa, Google's Assistant and Apple's Siri. In August, Microsoft and Amazon launched a long-awaited collaboration between Cortana and Alexa.
Microsoft didn't immediately respond to a request for comment.