Felt is reported to have suffered from congestive heart failure but the exact cause of his death at his home in Santa Rosa, Calif., was not immediately known. His daughter, Susan Felt, told the Post in a phone interview that he "slipped away," after falling asleep.
The mysterious key player in one of the nation's most famous political dramas, Felt insisted that his identity be kept secret for decades after he leaked damaging information about Nixon and his aides to Washington Post reporter Bob Woodward in the early 1970s.
A whirlwind of speculation consumed Washington for decades afterward as to the identity of the source Post editors' dubbed "Deep Throat." While some suspected that Felt was the source who connected the White House to the June 1972 break-in at the headquarters of the Democratic National Committee, he denied the accusations until finally coming forward in a May 2005 Vanity Fair article.
"I'm the guy they used to call Deep Throat," Felt told Vanity Fair writer John D. O'Connor. The revelation sparked a barrage of media attention.
"People will debate for a long time whether I did the right thing by helping Woodward," Felt wrote in his 2006 memoir, "A G-Man's Life: The FBI, `Deep Throat' and the Struggle for Honor in Washington." "The bottom line is that we did get the whole truth out, and isn't that what the FBI is supposed to do?"
In 2005, Vanity Fair's O'Connor described on the NewsHour the process by which Felt decided to come forward.
"Over time he has realized that Deep Throat was a true American hero," O'Connor said. "His family has talked to him about this and they have convinced him through their communication, their love, that Deep Throat is nothing to be ashamed of."
The Post's Woodward and partner Carl Bernstein relied on various unnamed sources in reporting on Watergate, but "Deep Throat" helped to keep them on track and confirmed key information. The Post won a Pulitzer Prize for its Watergate coverage.
Reporting by the Post and other news organizations on the White House's ties to the Watergate break-in and other political schemes forced Nixon's resignation in 1974.
More than 30 officials would ultimately plead guilty or be convicted, including Attorney General John Mitchell, who served 19 months for conspiracy, obstruction of justice and perjury.
Felt was also memorialized in "All the President's Men," a popular 1976 movie about the Watergate drama.
In the movie, the enduring image of "Deep Throat" is of a chain-smoking Hal Holbrook telling Woodward, played by Robert Redford, to "follow the money."
Born on Aug. 17, 1913 in Twin Falls, Idaho, Felt came to Washington as a Capitol Hill staff member and later worked at the Federal Trade Commission before joining the FBI in 1942.
Some critics speculated that Felt, a J. Edgar Hoover loyalist, was angry at being passed over when Nixon appointed an FBI outsider, L. Patrick Gray, to lead the FBI after Hoover's death.
"We had no idea of his motivations, and even now some of his motivations are unclear," Bernstein said of Felt's reasons for leaking the information, according to the Associated Press.