The software maker took on Randall Long, who led the FTC's investigation into Google's acquisition of AdMob, to be a lobbyist in its Washington office, according to both Google and Microsoft.
The hiring, first reported by the Wall Street Journal, is the latest salvo in Microsoft's ongoing battle with Google for dominance of the high-tech world.
During his work on Google mergers, Long, who was a deputy assistant director of an FTC section that assessed mergers for antitrust violations, read thousands of confidential documents ranging from sales figures and emails to internal strategy memos, the source said.
The FTC is currently looking into complaints that Google's search results favor the company's other services.
An FTC spokesperson said that agency rules would prohibit Long from appearing before the agency to discuss any matter that he worked on "personally and substantially."
He would have to obtain clearance from the FTC to discuss any Google matter that was pending when he still worked for the agency. But Long had not worked on the current antitrust probe of Google's business practices, according to an FTC source.
The problem for Long will be to ensure that he never discusses confidential Google data as part of his new job, which could be difficult because he has seen so much, said a Washington antitrust attorney who asked not to be named because he practices before the FTC.
"It smacks of ethical problems all over. It smells bad," he said.
But another attorney said lawyers often work for different companies, sometimes rivals, and are trained to not use confidential information from one case in another.
The head of Microsoft's Washington, D.C. office, Fred Humphries, said that Microsoft was looking forward to Long's arrival.
"His deep legal experience will provide important perspective on a range of issues that affect both consumers and the broader technology industry," he said in an email statement. (Reporting By Diane Bartz; Editing by Steve Orlofsky)