Unlike other defendants in the scandal that brought down Richard Nixon, Mr. Liddy refused to testify and drew the longest prison term.

G. Gordon Liddy, a cloak-and-dagger lawyer who masterminded dirty tricks for the White House and concocted the bungled burglary that led to the Watergate scandal and the resignation of President Richard M. Nixon in 1974, died on Tuesday in Mount Vernon, Va. He was 90.

His death, at the home of his daughter Alexandra Liddy Bourne, was confirmed by his son Thomas P. Liddy, who said that his father had Parkinson’s disease and had been in declining health.

Decades after Watergate entered the lexicon, Mr. Liddy was still an enigma in the cast of characters who fell from grace with the 37th president — to some a patriot who went silently to prison refusing to betray his comrades, to others a zealot who cashed in on bogus celebrity to become an author and syndicated talk show host.

As a leader of a White House “plumbers” unit set up to plug information leaks, and then as a strategist for the president’s re-election campaign, Mr. Liddy helped devise plots to discredit Nixon “enemies” and to disrupt the 1972 Democratic National Convention. Most were far-fetched — bizarre kidnappings, acts of sabotage, traps using prostitutes, even an assassination — and were never carried out.

But Mr. Liddy, a former F.B.I. agent, and E. Howard Hunt, a former C.I.A. agent, engineered two break-ins at the Democratic National Committee offices in the Watergate complex in Washington. On May 28, 1972, as Mr. Liddy and Mr. Hunt stood by, six Cuban expatriates and James W. McCord Jr., a Nixon campaign security official, went in, planted bugs, photographed documents and got away cleanly.

A few weeks later, on June 17, four Cubans and Mr. McCord, wearing surgical gloves and carrying walkie-talkies, returned to the scene and were caught by the police. Mr. Liddy and Mr. Hunt, running the operation from a Watergate hotel room, fled but were soon arrested and indicted on charges of burglary, wiretapping and conspiracy.

In the context of 1972, with Mr. Nixon’s triumphal visit to China and a steam-rolling presidential campaign that soon crushed the Democrat, Senator George S. McGovern, the Watergate case looked inconsequential at first. Mr. Nixon’s press secretary, Ron Ziegler, dismissed it as a “third-rate burglary.”

But it deepened a White House cover-up that had begun in 1971, when Mr. Liddy and Mr. Hunt broke into the office of the psychiatrist of Daniel Ellsberg, who leaked the Pentagon Papers to The New York Times, looking for damaging information on him. Over the next two years, the cover-up unraveled under pressure of investigations, trials, hearings and headlines into the worst political scandal — and the first resignation by a sitting president — in the nation’s history.

G. Gordon Liddy after his release from prison in Danbury, Conn., on Sept. 7, 1977.
Fred R. Conrad/The New York Times

Unlike the other Watergate defendants, Mr. Liddy refused to testify about his activities for the White House or the Committee to Re-elect the President, and drew the longest term among those who went to prison. He was sentenced by Judge John J. Sirica to 6 to 20 years, but served only 52 months. President Jimmy Carter commuted his term in 1977.

“I have lived as I believed I ought to have lived,” Mr. Liddy, a small dapper man with a baldish pate and a brushy mustache, told reporters after his release. He said he had no regrets and would do it again. “When the prince approaches his lieutenant, the proper response of the lieutenant to the prince is, ‘Fiat voluntas tua,’” he said, using the Latin of the Lord’s Prayer for “Thy will be done.”

Disbarred from law practice and in debt for $300,000, mostly for legal fees, Mr. Liddy began a new career as a writer. His first book, “Out of Control,” (1979) was a spy thriller. He later wrote another novel, “The Monkey Handlers” (1990), and a nonfiction book, “When I Was a Kid, This Was a Free Country” (2002). He also co-wrote a guide to fighting terrorism, “Fight Back! Tackling Terrorism, Liddy Style” (2006), and produced many articles on politics, taxes, health and other matters.

In 1980, he broke his silence on Watergate with his autobiography, “Will.” The reviews were mixed, but it became a best seller. After years of revelations by other Watergate conspirators, there was little new in it about the scandal, but critics said his account of prison life was graphic. A television movie based on the book was aired in 1982 by NBC.

Mr. Liddy found himself in demand on the college-lecture circuit. In 1982 he teamed with Timothy Leary, the 1960s LSD guru, for campus debates that were edited into a documentary film, “Return Engagement.” The title referred to an encounter in 1966, when Mr. Liddy, as a prosecutor in Dutchess County, N.Y., joined a raid on a drug cult in which Mr. Leary was arrested.

In the 1980s, Mr. Liddy dabbled in acting, appearing on “Miami Vice” and in other television and film roles. But he was better known later as a syndicated talk-radio host with a right-wing agenda. “The G. Gordon Liddy Show,” begun in 1992, was carried on hundreds of stations by Viacom and later Radio America, with satellite hookups and internet streaming. It ran until his retirement in 2012. He lived in Fort Washington, Md.

Mr. Liddy, who promoted nutritional supplements and exercised, was still trim in his 70s. He made parachute jumps, took motorcycle trips, collected guns, played a piano and sang lieder. His website showed him craggy-faced with head held high, an American flag and the Capitol dome in the background.

George Gordon Battle Liddy was born on Nov. 30, 1930, in Brooklyn to Sylvester J. and Maria (Abbaticchio) Liddy. He grew up in Hoboken, N.J., a fearful boy with respiratory problems who learned to steel himself with tests of will power. He lifted weights, ran and, as he recalled, held his hand over a flame as an act of self-discipline. He said he once ate a rat to overcome a repulsion, and decapitated chickens for a neighbor until he could kill like a soldier, “efficiently and without emotion or thought.”

Like his father, a lawyer, Gordon attended all-male St. Benedict’s Prep School in Newark and Fordham University in the Bronx. After graduating from Fordham in 1952, he took an Army commission with hopes of fighting in Korea, but was assigned to an antiaircraft radar unit in Brooklyn. In 1954, he returned to Fordham and earned a law degree three years later.

In 1957, he married Frances Ann Purcell. The couple had five children. Along with his son Thomas and daughter Alexandra, he is survived by another daughter, Grace Liddy; two other sons, James Liddy and Raymond J. Liddy; a sister, Margaret McDermott; 12 grandchildren and two great-grandchildren. Mr. Liddy’s wife died in 2010.

From 1957 to 1962, Mr. Liddy was an F.B.I. field agent in Indianapolis, Gary, Ind., and Denver, and a supervisor of crime records in Washington. He then worked in patent law for his father’s firm in New York for four years. He joined the Dutchess County district attorney’s office as an assistant prosecutor in 1966.

In 1968, he began a dizzying, three-year rise from obscurity in Poughkeepsie to the White House. Challenging Hamilton Fish Jr. in a primary for the Republican nomination for Congress in what was then New York’s 28th District, he fell short, but his consolation prize was to take charge of the Nixon campaign in the mid-Hudson Valley, which the president won handily.

His reward was a job at the Treasury Department in Washington as a special assistant for narcotics and gun control. He helped develop the sky marshal program to counteract hijackers. Impressed, Egil Krogh, a deputy assistant to the president, recommended him in 1971 to John N. Mitchell, the attorney general, who recommended him to John D. Ehrlichman, the president’s domestic policy adviser.

Mr. Nixon, furious over the disclosure of the Pentagon Papers, had directed Mr. Ehrlichman to set up the “plumbers” to plug leaks and punish opponents. Among other operations, Mr. Liddy and Mr. Hunt, who were in charge of the unit, broke into the Beverly Hills office of Daniel Ellsberg’s psychiatrist, Dr. Lewis Fielding, for material to discredit the military analyst. They found none.

When the group was disbanded in 1971, Mr. Liddy went to work for the Nixon campaign. His title was general counsel, but his role was to plot more dirty tricks under a code name, “Gemstone.” They included kidnapping radicals who might disrupt the Republican convention, sabotaging the air-conditioning at the Democratic convention in Miami, hiring prostitutes to entrap Democrats with hidden cameras, and killing the syndicated columnist Jack Anderson, whom Mr. Liddy viewed as a national security risk.

But only the Watergate burglaries were carried out. It was a piece of tape over the lock on a garage-level door that tripped up the burglars. A security guard called the police, and a crackling walkie-talkie in Mr. Liddy’s hotel room told the tale:

“It looks like … guns!” one burglar whispered. “They’ve got guns. It’s trouble.”

The team’s lookout in an apartment across the street, broke in: “Now I can see our people. They’ve got their hands up. Must be the cops. More cops now. Uniforms … ”

“They got us!”

It was all over. Mr. Liddy and Mr. Hunt went home. It was 3 a.m. when Mr. Liddy got in, and his wife awoke. “Anything wrong?” she asked.

“There was trouble,” he said. “Some people got caught. I’ll probably be going to jail.”

Neil Vigdor contributed reporting, and Jack Begg contributed research.