Julie London was born Nancy Gayle Peck in Santa Rosa, California, on September 26, 1926, the only child of Jack and Josephine Peck, a vaudeville song-and-dance team. The family moved to San Bernardino in 1929, where she made her singing debut at three on her parents' radio program, and to Hollywood in 1941. She and her mother both admired Billie Holiday. In 1943, as a teenager working as an elevator operator at a clothing store on Hollywood Boulevard, she was noticed by the talent agent Sue Carol, who arranged a screen test. She made her film debut under the name Julie London in Nabonga (1944) while still in high school, graduated from Hollywood Professional School in 1945, and after a Warner Bros. contract that brought Task Force (1949) and Return of the Frontiersman (1950) took the lead in The Fat Man (1951).

In 1947 she married the actor and producer Jack Webb, who went on to create Dragnet. The two shared a love of jazz, had two daughters, Stacy and Lisa, and divorced in 1954. Webb directed and starred in Pete Kelly's Blues (1955), a film about a 1920s jazz bandleader with Peggy Lee and Ella Fitzgerald in the cast, and London had helped her old schoolmate Arthur Hamilton get work writing for Webb's production company. For that film Hamilton wrote a song for Fitzgerald. Webb objected to one word in the lyric, Hamilton would not change it, and the song was dropped. A few nights later Hamilton played it for London at her house, and she asked to record it. It was "Cry Me a River."

After the divorce she tried singing for a living. The jazz pianist and songwriter Bobby Troup, whom she would marry in 1959, helped book her into a small Hollywood club in the summer of 1955, without an audition. Barney Kessel played guitar behind her, and the bassist Ray Leatherwood, who took over from Ralph Pena partway through the run, completed a trio with no drums and no piano. Si Waronker, who had just started the Los Angeles label Liberty Records, came to hear her and signed her as one of the label's first artists. Troup produced the first sessions, held in Hollywood in August 1955. "Cry Me a River" went on at the end of one session as the only new song among the standards from her club set. Neither Kessel nor Leatherwood had seen the music, and the arrangement was sketched on the spot. Liberty issued it as a single in the fall of 1955, backed with "S'Wonderful." A four-song EP for Bethlehem Records, with Troup's own quintet accompanying her, also dates from 1955. The Liberty single stayed on the pop singles chart for five months, was the label's first hit, reached No. 9 in the United States and, in 1957, No. 22 in the United Kingdom, and by the Library of Congress's count sold more than 800,000 copies.

The album, Julie Is Her Name (Liberty LRP-3006, December 1955), kept the same instrumentation of voice, guitar and bass and reached No. 2 on Billboard's album chart. Billboard named her the most popular female vocalist of 1955, 1956 and 1957. She sang "Cry Me a River" on screen in The Girl Can't Help It (1956) and made her New York nightclub debut in January 1956. Troup went on to produce most of her Liberty albums of the decade and supplied songs, among them the title track of Lonely Girl (1956) and "The Meaning of the Blues," which she recorded on About the Blues (1957) and which Miles Davis cut the same year with Gil Evans's orchestra on Miles Ahead. That album used an eighteen-piece band arranged by Russ Garcia, whose listed players include Willie Smith on alto saxophone, Maynard Ferguson on trumpet, Kessel on guitar and Shelly Manne on drums. Julie (1957), with Jimmy Rowles conducting the orchestra, is credited with players including Benny Carter, Buddy Collette, Bud Shank, Georgie Auld, Pete Candoli and Jack Sheldon, and Leatherwood on bass. Julie Is Her Name, Volume II (1958) paired her with the guitarist Howard Roberts and Red Mitchell on bass, and on Swing Me an Old Song (1959) she was accompanied by Rowles and his orchestra. Calendar Girl (1956), Make Love to Me (1957) and London by Night (1958) used larger orchestral settings, and the provocative covers of those albums drew as much attention as the music.

In March 1960 she recorded Julie...At Home in her own living room, with Rowles on piano, Al Viola on guitar, Don Bagley on bass, Earl Palmer on drums and Emil Richards on vibraphone, and Bob Flanigan of the Four Freshmen sitting in on trombone for two tracks. Around Midnight, cut that August at Capitol Studios with Dick Reynolds arranging and conducting, put a larger band behind a program of "'Round Midnight," "Lush Life," "Misty" and "Black Coffee." Both albums are listed in the reference book 1,000 Recordings to Hear Before You Die.

Through the 1960s Liberty steered her toward the pop market under producers such as Snuff Garrett, though the jazz players kept turning up. Kessel played guitar on "I Loves You, Porgy" on Love Letters (1962). The End of the World (1963) reached No. 127 and The Wonderful World of Julie London (1963) No. 136 on Billboard's album chart, her last albums to chart there. In April 1964 she recorded In Person at the Americana at The Royal Box supper club in New York, with arrangements by Bagley, Jerome Richardson on reeds and Sal Salvador on guitar. In July 1965 she returned to a jazz setting for All Through the Night: Julie London Sings the Choicest of Cole Porter (issued in early 1966), recorded at Pacific Jazz Studios in Los Angeles over three days with the Bud Shank Quintet: Shank on alto saxophone and flute, Joe Pass on guitar, Monty Budwig on bass, Russ Freeman on piano and arrangements, and Colin Bailey on drums. Her last studio album, Yummy, Yummy, Yummy (1969), was a set of 1960s pop songs arranged by Tommy Oliver, and its title track reached No. 25 on Billboard's Bubbling Under chart. After it she stopped singing professionally, having lost much of her vocal control to years of heavy smoking and drinking.

In 1972 Webb, by then her ex-husband and still a close friend, cast her and Troup in Emergency!, which he produced. She played the nurse Dixie McCall and Troup played Dr. Joe Early, and she received a Golden Globe nomination in 1974. The series ran for six years and 128 episodes before it was cancelled in 1977, with television movies following through 1979. Her last recording was "My Funny Valentine" for the film Sharky's Machine (1981). She and Troup had three children, Kelly and the twins Jody and Reese. Troup passed away on February 7, 1999. London suffered a stroke in 1995, declined treatment after a lung cancer diagnosis in late 1999, and passed away on October 18, 2000, aged 74. She is buried beside Troup at Forest Lawn Hollywood Hills.

"Cry Me a River" was inducted into the Grammy Hall of Fame in 2001, was added to the Library of Congress's National Recording Registry, and ranks No. 48 on NPR's list of the 50 greatest jazz vocals. London has a star on the Hollywood Walk of Fame for her recordings. Lana Del Rey and Billie Eilish have named her as an influence.

London is a crossover case in the plainest sense: a film actress turned torch singer whose catalog divides between jazz-inflected small-group records and orchestrated traditional pop, with a run of 1960s pop covers at the end. AllMusic's biographer wrote that she lacked the range of Ella Fitzgerald or Sarah Vaughan and made up for it with restraint, softness and subtlety. She sang in a low, breathy contralto and, by her own account, needed to stay close to the microphone, a technique the engineers on her first sessions built into the sound. Her early records are famous for their sparseness: guitar and bass, sometimes a piano or a small band, and room around the voice.

The jazz side is documented in the people on the sessions. Barney Kessel and Ray Leatherwood on the debut, Howard Roberts and Red Mitchell on Volume II, Rowles and Bagley and Palmer on At Home, Bud Shank, Joe Pass and Russ Freeman on the Cole Porter album, and a repertoire that took in "Lush Life," "'Round Midnight," "Misty," "Basin Street Blues" and "I Loves You, Porgy." Her husband Bobby Troup was a working jazz pianist and the composer of "(Get Your Kicks on) Route 66," and he produced and wrote for her through the 1950s. She grew up admiring Billie Holiday, and the Library of Congress's essay on "Cry Me a River" notes that she was the only leading female vocalist of the 1950s without a big-band background, which left her free to sing the way she had on the club stand: quietly, and close in.