William Gargan was born William Dennis Gargan on July 17, 1905, in Brooklyn, New York. He was the younger brother of actor Edward Gargan, whose birthday July 17 he shared. His father was a detective, and his mother was a teacher. He graduated from St. James School in Brooklyn. On leaving school, Gargan became a salesman of bootleg whiskey to New York speakeasies and then joined a detective agency. While visiting his brother on a musical comedy stage, he was offered a stage job which he accepted. He began his stage career in Aloma of the South Seas. Gargan’s first movie was Rain later he played in Misleading Lady and had character roles in many Hollywood productions, including two appearances as detective Ellery Queen. He was cast in a number of stereotypical Irish parts in films playing policemen, priests, reporters, and blustering adventurers. In 1945 he played Joe Gallagher in The Bells of St. Mary’s, starring Bing Crosby and Ingrid Bergman. In 1935, Gargan went to England and made several movies there. In 1940, Gargan was nominated for the Academy Award for Best Supporting Actor for his role as Joe, the foreman, in They Knew What They Wanted. Gargan was best known for his role as private detective Martin Kane in the 1949–51 radio-television series, Martin Kane, Private Eye, sponsored by U.S. Tobacco. He also appeared as a private detective in the NBC radio show Barrie Craig, Confidential Investigator, which ran from 1951 to 1955.

Gargan starred in 39 episodes of The New Adventures of Martin Kane, a syndicated series premiering September 14, 1957, and distributed in Europe by United Artists Television for Ziv Television Programs. Gargan’s acting career came to an end in 1958 when he developed throat cancer, and doctors were forced to remove his larynx in 1960. Speaking through an artificial voice box, Gargan became an activist and spokesman for the American Cancer Society, often warning about the dangers of smoking. In 1954, Mutual of Omaha presented its annual Criss Award to Gargan for “his inspirational self-rehabilitation efforts and his outstanding contributions to established rehabilitation programs.” No longer able to act, he formed William Gargan Productions, making traditional films and television movies in Hollywood. He died aged 73 on February 17, 1979, on a flight between New York and San Diego. He was buried at Holy Cross Cemetery, San Diego, California