Jack Klugman, an Emmy Award-winning actor who excelled in disarming everyman roles, notably in the sitcom “The Odd Couple” as a slovenly sportswriter and in the police drama “Quincy, M.E.” as a principled medical examiner, died Dec. 24 in Los Angeles. He was 90.

His son, Adam Klugman, confirmed the death to the Associated Press. The cause was not immediately determined.

Mr. Klugman became a household name with his comic role in “The Odd Couple,” for which he received two Emmys during the show’s run on ABC from 1970 to 1975.

The series was adapted from a 1965 Neil Simon comedy about mismatched New York oddballs: a compulsively tidy photographer named Felix Unger who rooms with his best friend and fellow divorced man, an unkempt sportswriter named Oscar Madison. The play was a Broadway hit with Art Carney as Felix and Walter Matthau as Oscar. Matthau and Jack Lemmon co-starred in a 1968 film version.

Mr. Klugman stepped in as a replacement for Matthau on Broadway soon after the play opened. He was signed to play the role of Oscar in the television series, with Tony Randall cast as Felix.

The show was taped before a live audience, and the use of a laugh track was forbidden. If the crowd didn’t react to the jokes, the actors would ad-lib scenes until they found something the audience liked. When the show was edited, only the funniest takes were used.

Mr. Klugman won the first of his three Emmy Awards in 1964 for “The Defenders,” a courtroom drama. He played an actor whose old membership in a Communist-front organization had ugly repercussions during the Hollywood blacklist of the 1950s.

Before turning to television, Mr. Klugman was an established character actor in films, with roles in Sidney Lumet’s courtroom drama “12 Angry Men” (1957) and Blake Edwards’s “Days of Wine and Roses” (1962), in which he played an alcoholic. He also appeared in “Goodbye, Columbus” (1969) as the father of a spoiled young woman played by Ali MacGraw.

In 1976, Mr. Klugman returned to television in “Quincy, M.E.,” as a medical examiner in the Los Angeles County coroner’s office who used forensic science to get to the bottom of suspicious deaths. “Quincy” aired on NBC until 1983 and netted Mr. Klugman four Emmy nominations for lead actor in a dramatic series. He described “Quincy” as a precursor to later crime-scene investigation shows, which he said “just took what we did and made it bloodier and sexier.”

Jacob Joachim Klugman was born April 27, 1922, in South Philadelphia to an impoverished Jewish family. He was in his teens when his father, a house painter, died. His mother became a hat maker to support her six children.

Mr. Klugman said he was initially drawn to acting after watching child actor Jackie Cooper in the tear-jerking boxing drama “The Champ” (1931). He later said he did not pursue acting seriously because he thought actors “had to be born to a certain station in life.”

Mr. Klugman said he got into acting after he returned from Army service during World War II and acquired serious gambling debts.

“I owed a loan shark, who was also a friend, some money,” he once told an interviewer. “I had to get out of town. Since I had the G.I. Bill, I remembered my brother knew a guy in the Army who had been to Carnegie Mellon University, so I went there.”

He studied acting at the Pittsburgh college (although one of his teachers advised him that his talents were more suited to truck driving). He later moved to New York to study at the American Theatre Wing and take small roles on television and on Broadway.

Mr. Klugman received a Tony Award nomination for his supporting role in the Broadway musical “Gypsy” (1959) as the boyfriend of an indomitable stage mother played by Ethel Merman. Often described more as a reliable than a dynamic performer, Mr. Klugman impressed reviewers with his steady work ethic.

Amid an otherwise scathing analysis of 1968’s “The Sudden & Accidental Re-Education of Horse Johnson,” in which Mr. Klugman played a warehouse employee who quit his job to seek life’s meaning, New York Times drama critic Clive Barnes applauded him for “giving the play everything he could short of a heart transplant.”

In 1953, Mr. Klugman married actress and comedian Brett Somers, best known as a game-show panelist on the “Match Game” in the 1970s. He and Somers separated in 1974 but never divorced. She died in 2007.

Mr. Klugman’s 18-year relationship with actress Barbara Neugass ended in 1992 and led to an ugly palimony suit that Neugass ultimately lost.

In 2008, Mr. Klugman married actress Peggy Crosby. She survives, along with two sons from his first marriage.

Mr. Klugman starred in the short-lived 1986 sitcom “You Again?” as a divorced supermarket manager. The next year, he returned to Broadway in Herb Gardner’s Tony-winning “I’m Not Rappaport” as a passionate octogenarian socialist. His co-star was Ossie Davis, with whom Mr. Klugman made his stage debut nearly 40 years earlier in an Equity Library Theater production, “Stevedore.”

In 1989, a surgical procedure to remove cancer from his larynx left Mr. Klugman virtually unable to speak. Working with a vocal coach, he regained his speaking ability and performed with Randall in a 1991 stage revival of “The Odd Couple.” The two filmed a 1993 TV movie, “The Odd Couple: Together Again” and performed together in many stage productions, including Simon’s “The Sunshine Boys.”

Mr. Klugman credited Randall, who died in 2004, with motivating him to persevere after the loss of his voice. Mr. Klugman became a spokesman for the American Cancer Society.

Television made him a wealthy man, and he invested millions in breeding horses.

He told a reporter he felt closest to his character in “The Odd Couple.”

“I am Oscar; I didn’t have to play him,” Mr. Klugman once said. “If I had my druthers, it would be to live my life carefree, and that was Oscar Madison’s philosophy, to simplify your life and enjoy. I’m simple, sloppy, a womanizer — or at least I was. I had all Oscar’s vices.”