Felt, who brought down Richard Nixon in the infamous Watergate scandal, was suffering from congestive heart failure, family fried John D. O'Connor told the Associated Press.
(O'Connor wrote the Vanity Fair article that unmasked Deep Throat, putting an end to one of the most enduring mysteries of the 20th century.)
A former second-in-command at the FBI, Felt leaked damaging information linking Nixon and his aides to the 1972 break-in at the Democratic National Committee headquarters. Although he steadfastly denied being the source for Washington Post reporter Bob Woodward, he finally came clean in 2005.
"I'm the guy they used to call Deep Throat," Felt said at the time.
Uncomfortable LegacyIn recent years, Felt – seen by some as a traitor and some as a hero – struggled to come to grips with his place in history.
"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?"
Woodward and his partner Carl Bernstein went on to win a Pulitzer Prize for their coverage of the Watergate scandal.
Felt is survived by two children, Joan and Mark Felt Jr., and four grandchildren. His wife Audrey died in 1984.