He was born in Hoboken, New Jersey and educated at Fordham University. Liddy graduated in 1952 and joined the US Army, serving for two years as an artillery officer during the Korean War. He returned home in 1954 to study law at Fordham. Graduating in 1957, he went to work for the FBI under J. Edgar Hoover. Also in 1957 he married Frances Ann Purcell.
He left the FBI in 1962 and worked as a lawyer in New York City and Dutchess County, New York. In 1966 he organized the arrest and unsuccessful trial of Timothy Leary. He ran unsuccessfully for the post of District Attorney and then for the House of Representatives in 1968. But he used his political profile to run the presidential campaign of Richard Nixon in the 28th district of New York.
In 1971, after serving unsuccessfully in several positions in the Nixon administration, Liddy was moved to Nixon's 1972 campaign, the Committee to Re-elect the President, also known as CREEP, in order to extend the scope and reach of the White House Plumbers unit. At CREEP, Liddy concocted several far-fetched plots intended to embarrass the Democratic opposition. Most were rejected, but one caught the eye of Nixon operatives in the White House, and with Nixon's approval, Liddy led and then botched the ill-conceived break-in of the Democratic National Campaign headquarters in the Watergate complex.
For his role in the Watergate scandal, which he coordinated with E. Howard Hunt, Liddy was convicted of conspiracy, burglary, and illegal wiretapping and served five years in prison before being pardoned by President Jimmy Carter.
In 1982, Liddy published an autobiography, titled Will, which sold more than a million copies and was made into a television movie. He joined the talk circuit, eventually landing a syndicated radio program that espouses extremely conservative views and is characterized by Liddy's provocative, sometimes wacky style.
He first appeared as an actor on a made-for-TV movie called The Highwayman. He also appeared on the TV series Miami Vice and in the movies When Nature Calls (1985), Super Force (1990), Street Asylum (1990) and Adventures in Spying (1992).