Based in Vancouver, Worldwide is far smaller than Microsoft but is expected to grow quickly as communications companies switch to fiber-optic communications networks.
Worldwide announced the appointment just minutes after Microsoft said Mr Maffei had resigned and would be replaced by company insider John Connors.
Mr Maffei, 39, has no direct experience as a telecoms company executive but led Microsoft's strategic partnerships and investments with AT&T, Telewest and others. He said in a statement: "Worldwide Fiber is very well positioned to capitalise on the amazing growth in Internet and data traffic."
Though Microsoft has a $600bn market value and has created two of America's largest fortunes, those of Bill Gates and Steve Ballmer, it can't offer the rewards of a hot start-up. Mr Maffei, who will stay at Microsoft for several weeks to ensure a smooth handover, joins Rob Glaser, of RealNetworks, Peter Neupert of Drugstore.com and Naveen Jain of Infospace.com on the list of managers who have left to head other firms. All won large amounts of equity after leaving, inspiring Microsoft to consider a Net tracking stock as a way to halt the brain drain.
Worldwide, founded in 1998, is completing a 22,000-mile fiber-optic network in the US and Canada, and is constructing a trans-Atlantic network connecting Boston and Halifax with Liverpool and Dublin.