Sinofsky has joined as a board partner, a role in which he will represent Andreessen Horowitz on the board of the companies it invests in. The position, he wrote Thursday on his personal blog, isn't a full-time job.
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Sinofsky first made contact with Andreessen Horowitz late last year and was captivated by what he viewed as the firm's commitment to "fostering innovation, product creation and working with product-focused entrepreneurs."
In his blog post, he profusely professed great admiration for the firm, its employees and leaders and for "the values clearly articulated and lived day to day by everyone at the firm."
In turn, Sinofsky hopes to contribute with his skills and extensive experience with product development at Microsoft, which culminated with his leadership of the development of the controversial Windows 8 OS.
A product of historical importance to Microsoft, Windows 8 has been a lightning rod for criticism. Formally launched in October of last year, Windows 8 has been blasted for its radically redesigned "Modern" interface, which is based on tile icons optimized for touchscreens and meant to help the OS gain traction in the tablet market.
Still, Apple's iOS and Android continue to dominate the tablet OS market, with Windows 8 a minor player.
Microsoft is busy working on a major revision of the OS called Windows 8.1 that attempts to address many of the objections from consumers and enterprise customers, including the removal of the Start button and the bumpy interplay between the Modern interface and the more traditional Windows 7-like desktop, included to run legacy Windows applications.
In short, the harshest critics maintain that Microsoft tried to make an OS that works well on both desktops and tablets, and ended up with a product that works well in neither. By contrast, Apple has iOS for its iPads and iPhones, and MacOS for its desktop and laptop computers.
Sinofsky, the public face of the Windows 8 project, left Microsoft in mid-November, about three weeks after the launch of the OS, shocking many in the industry and leading to widespread speculation for the causes and motives behind his departure.
Officially, he and Microsoft said at the time that the decision to part was mutual and cordial, but analysts said there may have been tension between Sinofsky and others in the Microsoft executive team, and that the problems could have been exacerbated by the criticism leveled at Windows 8 since the first builds became publicly available.
Sinofsky also oversaw the development of the Microsoft-branded Surface tablets, which have struggled to gain traction, in particular the model that runs Windows RT, the Windows 8 version for ARM-based devices.
In December, Sinofsky disclosed that he would teach at the Harvard Business School this spring, and in Thursday's blog post he reiterated that he will continue being an Entrepreneur in Residence there, as well as "pursue other business and product development opportunities that arise."
In May of this year, he publicly defended Windows 8 sales, and in July struck a "retirement agreement" with Microsoft in which he received a payout for unvested stock. In return, he agreed to some concessions that, among other things, prevent him from working for certain Microsoft competitors and from speaking badly about Microsoft products.
Juan Carlos Perez covers enterprise communication/collaboration suites, operating systems, browsers and general technology breaking news for The IDG News Service. Follow Juan on Twitter at @JuanCPerezIDG.