Karrin Allyson was born Karrin Allyson Schoonover on July 27, 1963, in Great Bend, Kansas. Her father was a Lutheran minister and her mother a psychotherapist, teacher, and classical pianist. She grew up in Omaha, Nebraska, spent her last year of high school in San Francisco, and as a girl studied classical piano, sang in church and in musical theatre, and began writing songs. At the University of Nebraska Omaha she held a classical piano scholarship, majored in piano, and minored in French. She also sang lead in an all-female rock band called Tomboy, performed in a jazz swing choir, and led her own jazz group that played Omaha clubs.

After graduating in 1986 she moved to Minneapolis to concentrate on jazz, and in 1990 she settled in Kansas City, where her career took hold. There she met Bill McGlaughlin, a conductor and public radio host who became her partner and co-produced or assisted on most of her albums, and the guitarist Danny Embrey, who drew her into Brazilian music. Her debut, I Didn't Know About You, was recorded with Kansas City players and first issued in 1992 on KAS Productions; Concord Jazz picked it up in 1993, and she stayed with the label for a dozen albums through 2011. Her working band in those years centered on pianist Paul Smith, guitarists Embrey and Rod Fleeman, bassist Bob Bowman, and drummer Todd Strait.

The Concord albums of the 1990s moved between bebop-based standards, blues, and Brazilian and French songs. Sweet Home Cookin' (1994) was recorded partly in Hollywood in June 1993 with arrangements by pianist Alan Broadbent and solos from trumpeter Randy Sandke and tenor saxophonist Bob Cooper. Azure-Té (1995) and Collage (1996) were cut at Soundtrek Studios in Kansas City, and Daydream (1997) added vibraphonist Gary Burton on three tracks and Randy Brecker on flugelhorn and trumpet. In 1998 she and McGlaughlin moved to New York City, and From Paris to Rio (1999) gathered songs sung in French and Portuguese.

Ballads: Remembering John Coltrane, recorded in New York in November 2000, paired her with pianist James Williams, bassist John Patitucci, and drummer Lewis Nash, with tenor saxophonists Bob Berg and James Carter and soprano saxophonist Steve Wilson as guests. It brought her first Grammy nomination for Best Jazz Vocal Album. In Blue (2002) used Mulgrew Miller and Peter Washington; Wild for You (2004) turned to singer-songwriter material with pianist and producer Gil Goldstein; and Footprints (2006), a set of vocal versions of bop and hard bop pieces, featured Jon Hendricks and the Portland singer Nancy King, along with saxophonist Frank Wess. Nominations followed for Footprints, Imagina: Songs of Brasil (2008), 'Round Midnight (2011), and Many a New Day (2015), five in all.

After the Concord years she released Yuletide Hideaway (2013) on her own Kas label and made Many a New Day, a Rodgers and Hammerstein trio album with pianist Kenny Barron and Patitucci, for Motema; it was recorded on May 1 and 2, 2015. Some of That Sunshine (2018) was her first album of all original songs, with guests including saxophonist Houston Person and violinist Regina Carter. Shoulder to Shoulder: Centennial Tribute to Women's Suffrage (2019) put her at the head of an all-female sextet with pianist Helen Sung, bassist Endea Owens, drummer Allison Miller, trumpeter Ingrid Jensen, and alto saxophonist Mindi Abair, with guest vocals from Madeleine Peyroux, Kurt Elling, and Veronica Swift. A Kiss for Brazil followed on Origin Records on May 17, 2024, with pianist Vitor Gonçalves, drummer Rafael Barata, guitarist Yotam Silberstein, bassist Harvie S, and the Bahian singer and guitarist Rosa Passos on two songs.

Allyson received an honorary Doctor of Fine Arts degree from the University of Nebraska Omaha in 2022. Her own site describes her as a former head of the jazz vocal department at the Hartt School's Jackie McLean Institute of Jazz at the University of Hartford, and it announced a Solo-Duo-Trio workshop with the guitarist Freddie Bryant in Florence, Massachusetts, scheduled for August 3 to 7, 2026. She lives in New York City. She and McGlaughlin married in 2025 after thirty-five years together; he passed away on July 21, 2026, at 82.

Allyson sings in English, French, and Portuguese, and her repertoire runs from bebop and blues to bossa nova, Great American Songbook standards, and the singer-songwriter songs she grew up with. AllMusic's Scott Yanow singled out her lyrical ballad singing and her skill at turning instrumental jazz pieces into vocal numbers by means of scat and vocalese; on Footprints she and lyricist Chris Caswell wrote words for bop pieces such as Dizzy Gillespie's "Con Alma" and Wayne Shorter's "Footprints," and elsewhere she has sung Charlie Parker's "Yardbird Suite," Clifford Brown's "Joy Spring," and Thelonious Monk's "Ask Me Now." The Brazilian side of her work, begun in Kansas City in the 1990s, runs through From Paris to Rio, Imagina, and A Kiss for Brazil and draws on Antonio Carlos Jobim and Ivan Lins, whom she credits with capturing her ear, along with the singer Elis Regina. Her pop and folk roots, from her college rock band to a piano-bar repertoire, surface on Wild for You and on her covers of Joni Mitchell and James Taylor, and In Blue includes songs by Mose Allison, Blossom Dearie, Abbey Lincoln, and Oscar Brown Jr. A Chicago Tribune review of her debut praised her scat singing and her arranging, likening her sound in part to that of Cassandra Wilson, and AllMusic reports that a Playboy readers' poll listed her beside Ella Fitzgerald and Shirley Horn early in her career. The critic Gary Giddins described her timbre as a mix of coolness and grit. She also plays piano on her own records: she is credited at the keyboard on her debut, on several 1990s Concord albums, on Ballads, Imagina, Footprints, Yuletide Hideaway, and Some of That Sunshine, and AllMusic notes that on 'Round Midnight she played all the keyboards.