Born in London, England, the son of a British World War I hero Sir Sydney Turing Lawford and the former May Somerville Bunny, he spent his early childhood in France, and began acting at a young age. As a child he severely injured his arm - in his words, "attempting to run through a glass door." Doctors were able to save the arm, but the injury continued to bother him throughout his life. In fact, the injury was considered so damaging as to keep him from entering World War II but this turn of fate was probably the greatest boon to his career. At that time, Hollywood was infatuated with the heroic Englishmen and as war movies were being churned out by the dozens and American actors began to be drafted, Lawford put his considerable talents to work for the cause.

Prior to the war, Lawford had a gained a contract position with the MGM studios. Lawford's first major movie role was A Yank At Eton (1942). He played a snobby bully opposite Mickey Rooney. The picture was a smash hit, and Lawford's performance was widely praised. He won even greater kudos for his performance in The White Cliffs Of Dover (1944), in which he played a young soldier in World War II. MGM gave him another important role in The Picture of Dorian Gray (1945). Lawford also made Son Of Lassie (1945) and won a Modern Screen magazine readers poll as the most popular actor in Hollywood. His fan mail jumped to thousands of letters a week.

Lawford's busiest year as an actor was in 1946, when two of his films opened within days of each other: Cluny Brown (1946), and Two Sisters From Boston (1946). Lawford was recognized as the romantic lead on the MGM lot. He appeared with Frank Sinatra for the first time in the musical, It Happened In Brooklyn (1947). Lawford later admitted that the most terrifying experience of his career was the first musical number he performed. He also made his first comedy that same year: My Brother Talks To Horses (1947). It was in Good News (1947) that he won his greatest acclaim as an actor. He also got to dance and sing, and held his own against the other cast.

Lawford was given other important roles in MGM films over the next few years, such as On An Island With You (1948), Easter Parade (1948) and Little Women (1949 film) (1949). His first marriage was to Patricia Kennedy Lawford, sister of future President John F. Kennedy, in 1954. They had four children, including actor Christopher Lawford. Lawford became an American citizen in 1960, in time to vote for his brother-in-law in the presidential elections. Lawford, along with other members of the "Rat Pack," helped campaign for Kennedy and the Democratic Party.

Lawford had a reputation as a ladies' man and was reported to have had many affairs with famous ladies of movies, song, and politics including Ava Gardner, June Allyson, Lana Turner, Janet Leigh, Rita Hayworth, Dorothy Dandridge, Lucille Ball, Anne Baxter, Judy Holliday, Gina Lollobrigida, Judy Garland, Marilyn Monroe, Grace Kelly, Kim Novak, Jacqueline Kennedy Onassis, Lee Remick, Nancy Reagan, and Elizabeth Taylor just to name a few. Lawford introduced Marilyn Monroe before she sang her infamous Happy Birthday, Mr. President song. He and his brother-in-law Robert F. Kennedy are rumoured to have visited Monroe on the day she died, although this has never been confirmed. The Kennedy family distanced itself from Lawford as his antics proved to be an embarrassment. Patricia Kennedy Lawford eventually divorced him in 1966 due to his alcoholism and infidelity.

Lawford was close to Frank Sinatra for a number of years, appearing in several Rat Pack movies and stage acts. Sinatra, however, threatened him with bodily harm when he learned that Lawford had lunch with Ava Gardner. Lawford's friends managed to convince Sinatra that nothing was going on between Gardner and Lawford, but Sinatra refused to speak with Lawford for a number of years. The two were later reconciled, but Sinatra ultimately broke off the friendship after Lawford refused to act as a go-between for Sinatra and President Kennedy as their association had become controversial. Sinatra's feelings were such that one time, when he learned that Lawford was in the audience he was about to perform in front of, that he refused to come out until Lawford and his wife were removed from the audience.

Later in life, Lawford fell into drug and alcohol abuse. Such abuse, plus strained relationships with others and financial difficulties caused a great deal of stress on his increasingly fragile health. Lawford was reduced to doing television guest shots on such shows as Fantasy Island, Alfred Hitchcock Presents, Profiles In Courage, The Wild Wild West, I Spy, The Name Of The Game, The Jeffersons, The Love Boat, The Virginian, Bewitched, The Patty Duke Show, The Doris Day Show, and Hawaii Five-O. Besides sitcoms, he also guest-starred on variety shows such The Judy Garland Show and Rowan & Martin's Laugh-In, and game shows such as What's My Line?, Password, and Pyramid.

Lawford married his second wife, Mary Rowan, daughter of Dan Rowan, in 1971. They divorced in 1975. He was married to his third wife, Deborah Gould, from 1976 to 1977, and finally married his fourth wife and widow, Patricia Seaton, in 1984. Lawford died in Los Angeles on Christmas Eve 1984 of liver and kidney disease culminating in cardiac arrest at the age of 61.

His body was cremated and the ashes were inurned at Westwood Village Memorial Park Cemetery. His original burial location was near that of Marilyn Monroe. According to his son, actor Christopher Lawford, talking on Larry King's CNN talk-show on September 27, 2005, none of the Rat Pack members attended the funeral, though a number of the Lawford/Kennedy cousins came. Because of a dispute between the family and the cemetery, however, his remains were removed and then scattered in the Pacific Ocean off the coast of California.