Mark Felt (1913 - 2008) studied at the University of Idaho and the George Washington University Law School; he joined the FBI in 1942. In the 1970s Felt was second in the FBI hierarchy. In 1981, he was pardoned by President Ronald Reagan of the felony of violating civil rights in 1972 and 1973.
On 19th October, 1972, White House Chief of Staff H. R. Haldeman told Nixon a secret source had identified Mark Felt as someone who was leaking information about Watergate to the press. Nixon considered sacking Felt but Haldeman urged caution: "He knows everything that`s to be known in the FBI. He has access to absolutely everything... If we move on him, he'll go out and unload everything."
During the Watergate Scandal some people speculated that Mark Felt was Deep Throat. "It was not I and it is not I," Felt told Washingtonian magazine in 1974. In a press conference in August 1976 Felt denied once again being Deep Throat. He added that he would admit it if it was true as he thought it would have been his moral duty to remove a corrupt politician from power. However, he said, it was not possible to take credit for something he did not do.
In May, 2005, Felt's lawyer, John O'Connor, went public with the news. Felt was quoted as saying: "I don’t think being Deep Throat was anything to be proud of. You should not leak information to anyone." However, he added: "If you know your government is engaging in illegal and/or immoral acts, then you have an obligation to speak out that overrides confidentiality agreements and secrecy laws. It's never wrong to inform on serious criminal acts no matter who is perpetrating them." 
Mark Felt, at age 91, is finally admitting to that historic, anonymous role. Vanity Fair, 2005.