Mr. Klugman was already a decorated actor in 1970 when he began co-starring in “The Odd Couple,” a television sitcom adaptation of Neil Simon’s hit play about two divorced men — friends with antagonistic temperaments — sharing a New York apartment. (A film version was released in 1968 with Walter Matthau reprising his Broadway performance as Oscar.) Mr. Klugman had played the part before: he had replaced Mr. Matthau for a few months on Broadway and had originated the role in London.

He also had more than 100 television credits behind him, including four episodes of “The Twilight Zone” and a 1964 episode of the legal drama “The Defenders,” in which he delivered an Emmy Award-winning performance as a blacklisted actor.

In the movies he had been the nouveau-riche father of a Jewish American princess (Ali MacGraw) in “Goodbye, Columbus” (1969); a police colleague of Frank Sinatra’s in “The Detective” (1968); Jack Lemmon’s Alcoholics Anonymous sponsor in “Days of Wine and Roses” (1962); and a murder-trial juror, alongside Henry Fonda, in “12 Angry Men” (1957).

In his solo moment in that film, his character, known only as Juror No. 5, recalls growing up in a tough neighborhood and instructs his fellow jurors in the proper use of a switchblade, a key element in their deliberations.

The “Odd Couple” series made Mr. Klugman a celebrity, but not immediately. During its five-year run, it never cracked the Top 20 in the Nielsen prime time ratings. Some critics said Mr. Klugman and Mr. Randall were always operating in the long shadows of the actors who came before them in the roles: besides Mr. Matthau as Oscar, Mr. Lemmon (film) and Art Carney (Broadway) had played Felix. But after “The Odd Couple” went into seemingly perpetual reruns, it earned a huge new following.

Mr. Klugman won two Emmys for the show and Mr. Randall one, and they eventually became the Oscar and Felix most identified with the roles.

Mr. Klugman’s path to success was serendipitous. He was the youngest of six children of immigrants from Russia. Most sources indicate that his name at birth was Jacob, though Mr. Klugman said in an interview that the name on his birth certificate is Jack.

His father, Max, was a house painter who died when Jack was 12. His mother, Rose, was a milliner who worked out of the family home in hardscrabble South Philadelphia, where Jack grew up shooting pool, rolling dice and playing the horses. His interest in acting was kindled at 14 or 15 when his sister took him to a play, “One Third of a Nation,” a “living newspaper” production of the Federal Theater Project about life in an American slum; the play made the case for government housing projects.

After a stint in the Army — he was discharged because of a kidney ailment — Mr. Klugman returned to Philadelphia but racked up a debt to loan sharks who were so dangerous that he left town. He landed in Pittsburgh, where he auditioned for the drama department at Carnegie Tech (now Carnegie Mellon University).

“They said: ‘You’re not suited to be an actor. You’re more suited to be a truck driver,’ ” he recalled. But this was 1945, the war was just ending and there was a dearth of male students, so he was accepted. “There were no men,” he said. “Otherwise they wouldn’t have taken me in.”

After two years at Carnegie he left for New York, where he led the poverty-stricken life of an aspiring actor, taking bit parts in summer stock and hole-in-the-wall New York productions, occasionally selling pints of blood to pay the rent. He roomed for a while with Charles Bronson, who introduced him to vigorous exercise.

In 1953, Mr. Klugman married the actress Brett Somers. They separated in 1974 (she played Oscar Madison’s former wife on “The Odd Couple”) but were never divorced.

After having vocal cord surgery, Mr. Klugman was prepared to devote himself to raising racehorses, a longtime side pursuit; one of his horses, Jaklin Klugman, finished third in the 1980 Kentucky Derby.