While Cortana is embedded into the Windows 10 operating system and is available on smartphones, tablets and smart speakers, the assistant remains less popular than Apple's Siri and drives fewer specialty device sales than Amazon's Alexa or Alphabet's Google Assistant. But Soltero was a rising star inside Microsoft who was plucked up in Satya Nadella's first year as CEO.
The departure was announced to employees this week, according to ZDNet. Soltero's last day will probably be in December, according to a person familiar with the move. It's not clear what he'll do next.
Soltero tweeted about his departure after the ZDNet report.
"We are grateful for Javier's impact and strategic leadership over the last four years at Microsoft, and wish him well as he returns to his entrepreneurial roots," a Microsoft spokeswoman told CNBC in an email. "Bringing together all of our work in natural language, intelligence and assistance -- including Cortana -- enables us to innovate faster for developers, partners and customers."
Soltero, a former VMware executive, got to Microsoft in 2014 when the company acquired email app start-up Acompli for $200 million, according to media reports at the time. In the next few months Soltero was involved in bringing a modern version of its Outlook email and calendar app to Android and iOS, while drawing on some features of the Acompli app, like its Focused Inbox that prioritized certain email messages.
Outlook is a component of Office 365, the bundle of productivity apps for consumers and businesses. Business subscriptions to Office 365 represent part of Microsoft's Commercial Cloud lineup, which saw 47 percent revenue growth in the fiscal first quarter.
The Outlook app has been well received on operating systems that aren't owned by Microsoft.
Soltero took charge of Cortana in March, ahead of a larger corporate shakeup. In August Microsoft and Amazon launched an integration of Cortana and Alexa. Last holiday season Amazon said it had sold "tens of millions" of devices running Alexa.