Clancy Sigal. (ANTHEA SIEVEKING/CAMERA PRESS LONDON)

He was a street-corner communist agitator in the early 1940s, then a World War II soldier who stared down one of the chief architects of the Holocaust at a Nazi war-crimes trial.

In Hollywood, he was Humphrey Bogart’s agent before being blacklisted during the McCarthy era. He was a promising novelist in the 1960s, experimented with LSD in London and was the model for a character in one of the 20th century’s most celebrated novels.

In his 90 adventurous years, until his death July 16 in Los Angeles, Clancy Sigal led a life overflowing with action, famous names, firings, breakups and a vision of America — at once earnest and cynical — born of many years as an expatriate.

Among his several novels, one (“Going Away”) was a finalist for the National Book Award. Since its first U.S. publication in 1962, it has become a cult favorite and has been compared favorably to Jack Kerouac’s “On the Road” as a restless portrait of 1950s America.

“It was as if ‘On the Road’ had been written by somebody with brains,” critic John Leonard wrote in the New York Times about Mr. Sigal. “His intelligence is always ticking. His ear is superb. His sympathies are promiscuous. His sin is enthusiasm.”

Clancy Sigal in about 1960. (Peter Keen)

There were countless love affairs along the way, and Mr. Sigal was briefly part of the Paris intellectual world of Jean-Paul Sartre and Simone de Beauvoir, the author of the feminist manifesto “The Second Sex.” Mr. Sigal hoped to seduce de Beauvoir, but he failed in that amorous attempt and was hastily driven out of Paris, by some accounts, at the point of a gun.

He moved in 1957 to London, where he rented a room from writer Doris Lessing, who 50 years later won the Nobel Prize for literature. During their four-year affair, each of them furtively read the other’s diaries and notebooks. Mr. Sigal was clearly the basis for the character of Saul Green, a handsome “American lefty” who was the lover of Anna Wulf, the protagonist of Lessing’s 1962 novel “The Golden Notebook.”

Mr. Sigal spent 30 years in England, working as a journalist and BBC commentator before returning to the United States in the late 1980s. Rejuvenated in a city he once condemned as a place of “too many freeways, too much sun, too much abnormality taken normally,” he wrote screenplays and published several books, including a memoir about the woman who occupied his thoughts more than any other: his remarkable, cinematically colorful free spirit of a mother.

His story began in Chicago, where he was born Sept. 6, 1926, as Clarence Sigal, named for a friend of his mother’s, the lawyer Clarence Darrow, who was known for taking on challenging cases, including the Scopes “monkey trial” the year before Mr. Sigal was born.

In his youth, a co-worker mispronounced Mr. Sigal’s first name as Clancy, which he soon adopted. His name led many people to believe that he was half-Irish and half-Jewish. In fact, both parents were Jewish immigrants from Eastern Europe, although they never married.

His mother, Jennie Persily, roamed the country as a union organizer. His father, Leo Sigal, was also a union organizer but was married to another woman, with whom he had a separate family. The younger Mr. Sigal saw little of his father after the age of 13.

Mother and son traveled around the country together, as his mother tried to organize workers. He was 5 the first time he spent a night in jail with his mother.

In a 2006 memoir about his mother, “A Woman of Uncertain Character,” he recalled sitting in a Southern police station while his mother was being interrogated: “Ma recrosses her silk-stockinged legs, reaches into her purse, pulls out a Pall Mall, and takes her time lighting it. Blows a perfect smoke ring.”

The next morning, they were told to leave town.

Mr. Sigal was 15 when he joined the Communist Party — a move his mother opposed. He served in the Army during and immediately after World War II. In 1946, he went to the Nuremberg war-crimes trial with the aim of assassinating Hermann Göring, one of Adolf Hitler’s leading deputies. After his gun was seized at the courtroom door, Mr. Sigal could only engage in a staredown with Göring.

After the war, Mr. Sigal studied English at the University of California at Los Angeles, where he was an editor of the campus newspaper. He occasionally engaged in heated political debates with two other UCLA students, H.R. Haldeman and John Ehrlichman, who became top White House aides to Richard M. Nixon and were imprisoned for their roles in the Watergate scandal.

Mr. Sigal graduated in 1950, then went to work at the studio of Columbia Pictures, where he was fired by Columbia boss Harry Cohn for using studio equipment to make copies of radical leaflets, which he dropped over Los Angeles from an airplane. He then joined the talent agency of Sam Jaffe, where his clients included Bogart, Barbara Stanwyck and Donald O’Connor.

Two prospective clients he turned down were a “hillbilly singer” named Elvis Presley and a young “mumbling” actor, James Dean.

As Mr. Sigal’s political sympathies became known, he left Hollywood and traveled around the country by car, which became the subject of “Going Away,” which, “better than any other document I know,” Leonard wrote in the Times, “identified, embodied and re-created the postwar American radical experience.”

While living in England, he published “Weekend in Dinlock,” a slightly fictionalized account of the culture of a Yorkshire coal-mining community, often likened to George Orwell’s “The Road to Wigan Pier.

In the early 1960s, he developed a friendship with the charismatic LSD exponent R.D. Laing, whom he came to see as a manipulative cult leader. He drew a scathing portrait of Laing in a 1976 novel, “Zone of the Interior.”

On return visits to the United States, Mr. Sigal took part in civil rights organizing in the South and the 1963 March on Washington.

In the late 1980s, he settled in Los Angeles, where he taught journalism at the University of Southern California and other colleges.

In 1992, he published a novel about an expatriate American radical in England, “The Secret Defector.” With his second wife, Janice Tidwell, he was a screenwriter of “In Love and War,” a 1996 film starring Sandra Bullock and Chris O’Donnell about Ernest Hemingway’s experiences in World War I. He also helped write the screenplay of “Frida,” a 2002 biopic about Mexican artist Frida Kahlo, starring Salma Hayek, Alfred Molina and Antonio Banderas.

Mr. Sigal’s first marriage, to Margaret Walters, ended in divorce. Survivors include Tidwell, his wife of 24 years, who confirmed Mr. Sigal’s death from congestive heart failure, and their son, Joseph Sigal, both of Los Angeles.

In addition to the memoir of his mother, Mr. Sigal published a book about Hemingway and, last year, a memoir about his years in Hollywood, “Black Sunset.” He continued to publish essays until several weeks ago.

Tidwell said Mr. Sigal “talked incessantly about his mother,” whom he described in his memoir as “a warrior queen, a crazy bohemian.”

Like her son, she carried on multiple love affairs and was volatile, passionate and proudly radical.

“It seemed as if all that anger, resentment, and pent-up fury she could never articulate, for fear of being consumed by it,” Mr. Sigal wrote, “shot straight into my veins.”