The No.. 2 G-Man in the early '70s during the Nixon administration, a veteran FBI field agent, who had served since the early 1940s under the direction of J. Edgar Hoover, Mark Felt will be remembered by most Americans as "Deep Throat" (a code name attached to him by Bob Woodward at the Washington Post to conceal his identity). Felt apparently was not amused by Woodward's risque choice of sobriquet -- "Deep Throat" was also the title of a quasi-mainstream porn movie that seemed to be on everybody's lips at the time.
The Washington Post has a thorough obit-cum-retrospective. Here's a sampling:
In 1980, he was convicted of approving illegal "black bag" break-ins against of the families and friends of Weather Underground radicals. He was later pardoned by President Ronald Reagan.
In his 1979 book, "The FBI Pyramid From the Inside," co-authored with conservative writer Ralph de Toledano, Felt supported Hoover's bugging of the Rev. Martin Luther King during the Kennedy administration. He opposed Gray's decisions to hire women as FBI agents, to loosen the dress code and to ease the weight restrictions for FBI agents.
He came from the traditional crime-fighting FBI, having started with the agency in 1942. He unmasked a German spy in the United States, chased bank robbers and for years led what was known internally as the "goon squad," which monitored the performance of field agents. Even after he was promoted to deputy associate director in 1971, his reputation was that of a hard-line Hoover loyalist.
No one knows exactly what prompted Felt to leak the information from the Watergate probe to the press. He was passed over for the post of FBI director after Hoover's 1972 death, a crushing career disappointment.
Mark Felt was 95.