Peter Lawford, 61, a British-born actor was as well known for the family he married into and the friends he made as he was for his nearly 60 films and scores of television appearances, died of cardiac arrest yesterday at Cedars-Sinai Medical Center in Los Angeles.

He had been admitted to the hospital Dec. 16 with liver and kidney ailments and three days later went into a coma. Earlier this year, Mr. Lawford was treated at the Betty Ford Center in Rancho Mirage, Calif., for drug and alcohol abuse.

His first marriage, to the former Patricia Kennedy, a sister of President Kennedy, ended in divorce. In later years he became socially prominent on Hollywood's party circuit and a member of Sinatra's "Rat Pack," which included Dean Martin, Joey Bishop and Sammy Davis Jr. They appeared together in several pictures, including "Ocean's 11" and "Sergeants 3."

Mr. Lawford, a tall, dark-haired actor, was a bobbysoxers' idol in the 1940s and a supporting actor in the 1950s who could play the dashing man-about-town with some elan.

He began his movie career when he was chosen for a role in "Old Bill," a 1930 English film. As a teenager, he appeared in MGM's "Lord Jeff" and later received widespread critical notice for a small part in the legendary film "Mrs. Miniver" and for "A Yank at Eaton." He portrayed a series of juvenile roles in "Son of Lassie," "Two Sisters from Boston" and "The White Cliffs of Dover."

He reached his height as an actor before he was 30.

His later movies included "Easter Parade," "Little Women," "Advise and Consent," "The Longest Day," "The Oscar," "Exodus," "Buena Sera Mrs. Campbell," "Mrs. Parkington," "The Picture of Dorian Gray" and "Harlow."

In the 1950s, Mr. Lawford starred in the TV series "Dear Phoebe" and "The Thin Man."

He produced, and appeared regularly in, the "Doris Day Show" in the early 1970s, playing Day's romantic interest. He also had produced the TV series "The Patty Duke Show" and "Ellery Queen."

Mr. Lawford had made few acting appearances in the past decade, but he was playing a talent agent in the CBS-TV movie "Malice in Wonderland" when he fell ill. It is scheduled to be broadcast next year.

In a prepared statement, Sen. Edward M. Kennedy (D-Mass.) said, "The death of Peter Lawford is a special loss to all of us in the Kennedy family, and my heart goes out to his children. . . . We take comfort from the fact that we know he will also be missed by all of the people who enjoyed his many roles in films and on television. He was a dedicated and creative actor as well as a loving father and loyal friend to all of us, especially in the challenging days of the New Frontier," a reference to the Kennedy presidency.

"I've lost a good friend and a colleague for many years," Martin said in a statement issued through his agent.

Sinatra, in a statement issued by his public relations firm, said, "I'm deeply saddened."

Phyllis Kirk, Mr. Lawford's co-star in "The Thin Man" in the late 1950s, called him "a unique, gracefully, gifted, intelligent and hilariously funny person."

He married Patricia Kennedy in April 1954. They were divorced in 1966 on grounds of mental cruelty, with Mrs. Lawford winning custody of their four children. But during Kennedy's term in the White House, the Lawfords were familiar faces on the Washington social scene. However, Mr. Lawford, more at home with the Hollywood elite, said he often felt like "the bit player in the background" at Kennedy family functions. "I have learned to play my role as the offbeat brother-in-law," he once said.

Mr. Lawford said the Kennedy associations had little effect on his career. "Being related to the president of the United States is a very great honor," he said. "But it is not, and never will be, a career. . . . Nobody would be insane enough to advertise 'Starring JFK's brother-in-law,' and if he did, I trust nobody would be insane enough to buy a single extra ticket."

Mr. Lawford, who became an American citizen in 1960, was born in London on Sept. 7, 1923, the only son of Lt. Gen. Sir Sidney and Lady May Somervile Lawford. He was reared at the various billets to which his father was posted, including Paris, Monte Carlo, Nice, Cannes, Sydney, Melbourne, New York, Colombo, Tahiti, Barcelona, Nassau, Bombay and Panama City.

Mr. Lawford's second marriage was to Mary Rowan, daughter of comedian Dan Rowan, in 1971. The bride was 21, Lawford, 48. She filed for a divorce in June 1973. At the time of his death, he was married to the former Patricia Seaton.

In addition to his wife, survivors include the four children from his marriage to Patricia Kennedy: Cristopher, 29; Sydney, 28; Victoria, 26, and Robin, 23.