The Harvard-trained Indian-American lawyer, who departed last week as NSA general counsel, arrived at the spy agency three years ago following stints at the White House, the Justice Department and the 9/11 Commission.
Soon enough, he found himself consumed with helping manage the external and internal responses to the disclosures of former agency contractor Edward Snowden.
De's outsider status, his White House ties and his willingness to engage with skeptics helped guide the digital spy agency through the biggest crisis in its history, observers say.
The agency also made him available for background chats with reporters.
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De's understated approach served him and the agency well, people who dealt with him say, even as he defended the programmes.
"I think he was the right person at the right time for NSA," said David Medine, chairman of the Privacy and Civil Liberties Oversight Board, who led investigations into leaked surveillance programmes that both bashed and backed the agency.
President Barack Obama has tinkered around the edges but left most of the surveillance programmes in place, rejecting some of the reforms suggested by his own task force. By all accounts, De, 41, played a role in preserving the status quo, much to the chagrin of critics.
In an interview with The Associated Press last week, De took issue with that view. He described an NSA completely at odds with the free-wheeling, all-seeing behemoth depicted by Snowden: a rule-bound, highly regulated entity that treats the private information of Americans with utmost care.
De is one of the highest-ranking Indian-Americans in government. His parents, both doctors, immigrated to Philadelphia with USD 16 to their names, he said. He is returning to his old law firm, Mayer Brown, and his replacement has not yet been named.