Steven Sinofsky, the executive in charge of Microsoft’s Windows 8 operating system and the driving force behind the new OS, is leaving the company, Microsoft announced late Monday.

Sinofsky was also the public face for Windows 8 and its new Metro interface, posting constant updates in a Windows 8 blog that charted its development. His last post, fittingly, was entitled “Updating Windows 8 for General Availability.” The OS was officially launched at the end of last month.

Sinofsky’s departure is effective immediately, Microsoft said. The company will promote Julie Larson-Green, who was a lead engineer on Windows 7, to lead all Windows software and hardware engineering. CFO Tami Reller will take on the added duty of managing the business side of Windows.

Melissa J. Perenson Steven Sinofsky, Microsoft’s president of Windows and Windows Live, left the company on Monday.

Microsoft didn’t say why Sinofsky left. In a statement, CEO Steve Ballmer thanked him for his work and added, somewhat ambiguously, that the company must “continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings.”

According to the All Things D blog, there was growing tension between Sinofsky and other members of the Microsoft executive team, who didn’t see him as enough of a team player. But Microsoft’s official position is that the decision was a mutual one.

Sinofsky’s official title was President of Windows and Windows Live. He was the executive who demonstrated Microsoft’s Surface tablet for the first time at an event in Los Angeles this year. And it was Sinofsky who disclosed to the media two years ago Microsoft’s plans to develop a version of Windows for ARM-based processors.

The timing of Sinofsky’s departure is surprising, given that it comes just weeks after the launch of Windows 8. Microsoft’s Windows 8 launch event was seen as underwhelming in some quarters, as it was a missed opportunity to connect with customers on changes to the new operating system.

That was hardly the only bump with the Windows 8 debut. After a strong initial weekend of upgrades to Windows 8, some reports indicate that customers are responding slowly to Windows 8, particularly with some of its interface changes.

Whatever the reasons for his departure, Sinofsky had only good things to say about his former employer of 20-plus years.

“It is impossible to count the blessings I have received over my years at Microsoft,” he said in a statement. “I am humbled by the professionalism and generosity of everyone I have had the good fortune to work with at this awesome company.”

PCWorld’s Melissa J. Perenson contributed to this report.

Updated at 7:45 p.m. PT to include more background on the recent Windows 8 launch.