The most recent head of Windows at Microsoft is getting into investing — specifically, venture capital and private equity.

Terry Myerson is becoming a venture partner at Madrona Venture Group and an operating executive at the Carlyle Group. Both roles are part-time. He'll continue to be based in Seattle.

He spent more than 21 years at Microsoft, and he became one of the 16 most important people at the company — one 2013 article described him as the single most important man there — as executive vice president for the Windows and Devices Group. In addition to Windows, he was also in charge of HoloLens, Surface, Xbox and other products.

In March, four years after Microsoft named Satya Nadella as CEO, the company announced that it was splitting up the Windows and Devices Group and putting people from Myerson's organization under two other leaders: Rajesh Jha and Scott Guthrie.

"I think a big part of it was Microsoft right now is going through some incredible growth," Myerson said in an interview with CNBC on Monday. "It's doing incredibly well. Satya is an incredible leader, and it really was a good time to step out."

He also worked at Microsoft as the company faced challenges, including an antitrust crackdown from the U.S. government and the stagnation of its stock price. Some people left during the tougher times, and their departures weren't easy to stomach, Myerson said. But under Nadella things have improved. For one thing, Microsoft's stock has tripled.

"It was both a good time to make a change and I was ready for a change," he said. "I'm so proud of what that company is doing right now and how well it's doing. It's nice to step away on a high note."

Leaving during difficult times means leaving others behind, and he didn't want to be one of those people, he said. By then it was a matter of whether he wanted to stick around for another decade. Ultimately, he felt that it was good to have other leaders come in and lead the next wave of growth, he said.

So he took a few months to spend time with friends and family members. He did a half-Ironman. He picked up some piano skills. But now he's getting back into the groove of work, a few weeks after his official day as a Microsoft employee.

"Candidly, I missed being part of a team," Myerson said. "I missed being part of the value-creation process."

At Carlyle, he'll be looking to draw on his experience building software at scale to help bigger companies as the private equity firm invests hundreds of millions of dollars, and at Madrona he's excited about working with entrepreneurs with innovative ideas and investing much smaller amounts, he said.

By entering venture capital, Myerson, who led Windows 10, is following the same path that two Windows leaders took before him. Brad Silverberg, who was responsible for Windows 95, co-founded Ignition Partners and Fuel Capital. Steven Sinofsky led the development of Windows 7 and Windows 8, and now he's a board partner at Andreessen Horowitz.

At Madrona he's working alongside another former Microsoft executive, S. "Soma" Somasegar, who was once in charge of the company's developer division.

"In the last couple of weeks that I have been working with Terry here, it has been fantastic to see a high level of intellectual curiosity and willingness to lean in and work with entrepreneurs that Terry has been demonstrating," Somasegar told CNBC in an email.

Myerson said he looks up to investors like John Doerr of Kleiner Perkins, who was active when Myerson was himself an entrepreneur, and Benchmark's Bill Gurley. When it comes to a single deal, he said he appreciated Andreessen Horowitz's funding of GitHub.

"I just love how they went all in. They just owned the investment. That's just great," Myerson said. Microsoft's $7.5 billion acquisition of GitHub, he said, was also "great."

And open-source software, Myerson said, is a secular trend. "It's here to stay," he said.

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