Julie London was born Gayle Peck in Santa Rosa, California, in 1926, the daughter of Jack and Josephine Peck, who were a vaudeville song-and-dance team. In 1929, when she was only 3, her family moved to San Bernardino, California, where Julie made her debut singing professionally on their public radio station. In 1941, when she was 14, the family moved to Hollywood, California. Shortly after that, she began appearing in movies. She graduated from the Hollywood Professional School in 1945.

In 1947 London married actor Jack Webb (of Dragnet fame). This pairing arose from their common love of jazz. They had two daughters, Stacy and Lisa Webb. London and Webb divorced in 1954. Daughter Stacy Webb was killed in a traffic accident in 1996.

In 1959 London married jazz composer and musician Bobby Troup, and they remained married until his death in 1999. They had one daughter, Kelly Troup, who died in 2002, and twin sons, Jody and Reese Troup. Jody Troup died in 2010.

London began singing under the name Gayle Peck in public in her teens before appearing in a film. She was discovered by talent agent Sue Carol (wife of actor Alan Ladd), while working as an elevator operator. Her early film career, however, did not include any singing roles.

London recorded 32 albums in a career that began in 1955 with a live performance at the 881 Club in Los Angeles. Billboard named her the most popular female vocalist for 1955, 1956, and 1957. She was the subject of a 1957 Life cover article in which she was quoted as saying, "It's only a thimbleful of a voice, and I have to use it close to the microphone. But it is a kind of oversmoked voice, and it automatically sounds intimate.

London's debut recordings were for the Bethlehem Records label. While shopping for a record deal, she recorded four tracks that would later be included on the compilation album Bethlehem's Girlfriends in 1955. Bobby Troup backed London on the album, for which London recorded the standards "Don't Worry About Me", "Motherless Child", "A Foggy Day", and "You're Blasé".

London's most famous single, "Cry Me a River", was written by her high-school classmate Arthur Hamilton and produced by Troup. The recording became a million-seller after its release in December 1955 and also sold on reissue in April 1983 from the attention brought by a Mari Wilson cover. London performed the song in the film The Girl Can't Help It (1956), and her recording gained later attention in the films Passion of Mind (2000) and V for Vendetta (2006). The song "Yummy Yummy Yummy" was featured on the HBO television series Six Feet Under and appears on its soundtrack album. London's "Must Be Catchin'" was featured in the 2011 premiere episode of the ABC series Pan Am. Her last recording was "My Funny Valentine" for the soundtrack of the Burt Reynolds film Sharky's Machine (1981).

Other popular singles include "Hot Toddy", "Daddy", and "Desafinado". Recordings such as "Go Slow" epitomized her career style: her voice is slow, smoky, and playfully sensual.

Though primarily remembered as a singer, London also made more than 20 films. Her widely regarded beauty and poise (she was a pinup girl prized by GIs during World War II) contrasted strongly with her pedestrian appearance and streetwise acting technique (much parodied by impersonators). One of her strongest performances came in Man of the West (1958), starring Gary Cooper and directed by Anthony Mann, in which her character, the film's only woman, is abused and humiliated by an outlaw gang.

She performed on many television variety series and also in dramatic roles, including guest appearances on Rawhide (1960) and The Big Valley (1968). On May 28, 1964, London and her husband Bobby Troup recorded a one-hour program for Japanese television in Japan. London was glamorously dressed as she sang thirteen of her classic songs including Bye Bye Blackbird, Lonesome Road, and Cry Me A River. Her ex-husband, Webb, was executive producer for the series Emergency!, and in 1972 he hired both his ex-wife and her husband, Troup, for key roles. London received second billing as head nurse Dixie McCall while Troup received third billing as emergency-room physician Dr. Joe Early. She and her co-stars Robert Fuller, a familiar actor and best friend of London's, as head physician Dr. Kelly Brackett along with unfamiliar actors Randolph Mantooth as paramedic John Gage and Kevin Tighe as paramedic Roy DeSoto also appeared in an episode of the Webb-produced series Adam-12, reprising their roles. London and Troup appeared as panelists on the game show Tattletales several times in the 1970s. In the 1950s, London appeared in an advertisement for Marlboro cigarettes singing the "Marlboro Song" and in 1978 appeared in television advertisements for Rose Milk Skin Care Cream.

London suffered a stroke in 1995 and was in poor health until her death on October 18, 2000 (the day her husband, Bobby Troup, would have been 82), in Encino, California, at age 74. London was interred next to Troup in the Courts of Remembrance, Columbarium of Providence, at Forest Lawn-Hollywood Hills Cemetery in Los Angeles, California. Her star on the Hollywood Walk of Fame is at 7000 Hollywood Boulevard in Los Angeles.