W. Mark Felt, who was the No. 2 official at the F.B.I. when he helped bring down President Richard M. Nixon by resisting the Watergate cover-up and becoming Deep Throat, the most famous anonymous source in American history, died Thursday. He was 95 and lived in Santa Rosa, Calif.

His death was confirmed by Rob Jones, his grandson.

In 2005, Mr. Felt revealed that he was the one who had secretly supplied Bob Woodward of The Washington Post with crucial leads in the Watergate affair in the early 1970s. His decision to unmask himself, in an article in Vanity Fair, ended a guessing game that had gone on for more than 30 years.

The disclosure even surprised Mr. Woodward and his partner on the Watergate story, Carl Bernstein. They had kept their promise not to reveal his identity until after his death. Indeed, Mr. Woodward was so scrupulous about shielding Mr. Felt that he did not introduce him to Mr. Bernstein until this year, 36 years after they cracked the scandal. The three met for two hours one afternoon last month in Santa Rosa, where Mr. Felt had retired. The reporters likened it to a family reunion.

Mr. Felt played a dual role in the fall of Nixon. As a secret informant, he kept the story alive in the press. As associate director of the Federal Bureau of Investigation, he fought the president’s efforts to obstruct the F.B.I.’s investigation of the Watergate break-in.