Helen Forrest was born Helen Fogel in Atlantic City, New Jersey, on April 12, 1917, the daughter of Louis and Rebecca Fogel. Her father died of influenza while she was an infant, and the family later moved to Brooklyn. A piano teacher, Honey Silverman, heard her singing during lessons and steered her toward voice, and she left high school to look for work. She sang with her brother Ed's band in Atlantic City, took a new surname on the advice of people in the business, and in 1934, at seventeen, began singing on New York radio at WNEW. At WCBS she was billed as "Bonnie Blue" and "The Blue Lady of Song." A job at the Madrillon Club in Washington, D.C., kept her working for roughly two years.

Bandleader Artie Shaw saw her at the Madrillon and hired her in 1938, when Billie Holiday was leaving his band. For a short time the two women worked in the same band, and Forrest later said that when she learned Black performers were required to wait offstage until their turn, she insisted on doing the same. She made 38 singles with Shaw's orchestra. "They Say" was cut in New York on December 19, 1938, "Deep Purple" followed on March 12, 1939 and reached No. 17 that year, and "All the Things You Are" was recorded on October 26, 1939. Shaw broke up the band in November 1939.

In December 1939 she joined Benny Goodman, with whom she made 55 studio recordings, including "The Man I Love." Goodman was the most demanding leader she worked for, and she remembered writing for her voice as an afterthought: she mostly sang choruses of arrangements built for the band, while Goodman noodled on clarinet behind her. During that period she also stepped outside his orchestra for two ballads on Lionel Hampton's small-group Victor sessions in Hollywood, "I'd Be Lost Without You" on May 10, 1940 and "I Don't Stand a Ghost of a Chance With You" on July 17, 1940, with Nat King Cole at the piano and Oscar Moore on guitar. Her Goodman recording of "Taking a Chance on Love," cut on November 29, 1940 and reissued in 1943, topped the charts on its return. She left Goodman in August 1941, saying she wanted to avoid a nervous breakdown, and Peggy Lee took the vocal chair.

She then approached Harry James with one condition, that she be allowed to sing more than a single chorus. James had been looking for a more jazz-oriented singer, but the band voted for her. He slowed tempos and built arrangements around his trumpet and her voice, and the three years that followed produced her most famous records: "I Don't Want to Walk Without You" (Columbia 36478), a No. 1 pop hit in 1942; "I Had the Craziest Dream," recorded July 23, 1942 and topping the Billboard charts; and "I've Heard That Song Before," cut on July 31, 1942, the last day of recording before the 1942 to 1944 musicians' strike, and a No. 1 hit in early 1943. She appeared with the James band in the 1942 film Springtime in the Rockies, and Down Beat readers voted her the top female vocalist of both 1942 and 1943. She left James in late 1943, explaining that three years with a band was enough.

As a soloist she signed with Decca, and her first single for the label, "Time Waits for No One" (Decca 18600, 1944), reached No. 2 on the Hit Parade. From 1944 she co-starred with Dick Haymes on his CBS radio program, and the two made 18 duets for Decca between 1944 and 1946, ten of them Top Ten hits. "Long Ago (and Far Away)" (Decca 23317) peaked at No. 2, and "It Had to Be You" / "Together" and "I'll Buy That Dream" followed. She appeared in the films Bathing Beauty and Two Girls and a Sailor in 1944, moved to MGM for singles in 1947 and 1948, and headlined theaters and clubs. In July 1955 she rejoined James in Hollywood to sing four numbers on Harry James in Hi-Fi (Capitol W 654). Voice of the Name Bands (Capitol T-704, 1956) revisited her band-era repertoire with orchestras led by Billy May and Dave Cavanaugh, and in May 1957 she recorded Black Magic in Los Angeles with clarinetist Buddy DeFranco, pianist Jimmy Rowles, vibraphonist Victor Feldman and guitarist Howard Roberts.

Rock and roll cut into the market for singers of her kind, and by the end of the 1950s her recording career had slowed. In the early 1960s she sang with the Tommy Dorsey Orchestra under Sam Donahue, and she was working at Lake Tahoe with Frank Sinatra Jr. in 1963 when he was kidnapped. The nostalgia circuit kept her busy through the 1970s and 1980s: a 1977 television reunion with James and Haymes on The Merv Griffin Show led to the touring revue The Fabulous 40s in 1978, followed by The Big Broadcast of 1944 in 1979. She suffered a stroke in 1980 and recovered to keep performing. Her autobiography, I Had the Craziest Dream, appeared in 1982. In January 1983 she recorded Now and Forever for Stash in New York with a jazz rhythm section of Hank Jones, George Duvivier and Grady Tate, plus Frank Wess on tenor saxophone and flute, with arrangements by Hale Rood. Rheumatoid arthritis affecting her vocal cords ended her singing in the early 1990s, and a hearing loss that began with scarlet fever in childhood had worsened with age. She passed away from congestive heart failure on July 11, 1999, at the Motion Picture & Television Country House and Hospital in Woodland Hills, Los Angeles, at 82. She was inducted into the now-defunct Big Band and Jazz Hall of Fame in 2001.

Forrest was a traditional pop and swing singer, and her career belongs mostly to the dance-band business rather than to the jazz club. AllMusic's assessment, as quoted on Wikipedia, is that some listeners would not call her a jazz vocalist, but that she projected a lyric exceptionally well and was an excellent interpreter. Her sound was warm and direct, with clear phrasing and an emotional openness that suited ballads, and the bandleader and critic George T. Simon called her a wonderfully warm and natural singer. She sang in tempos that let the words land, which is what Harry James designed his arrangements around.

The jazz side of her career is real but is better described as proximity and repertoire than as improvisation. She sang for three of the biggest bandleaders in the swing idiom, voiced the Gershwin, Kern and Porter songbook that jazz players still treat as raw material, and recorded in small-group settings with instrumentalists who were first-rank jazz players: Lionel Hampton's Victor session with Nat King Cole in 1940, Buddy DeFranco's quintet in 1957, and the Hank Jones rhythm section in 1983. Down Beat readers named her the best female band vocalist two years running, in 1942 and 1943. Her recordings are commonly sorted under the traditional pop of the 1940s, and the tags on this page, Big Band, Swing and Vocal Jazz, describe the settings she worked in rather than a claim that she was a bebop-era improviser.