Junko Onishi (大西順子) was born in Kyoto on April 16, 1967, and grew up in Tokyo, where she began classical piano lessons as a small child. The turn toward jazz came at Tokyo Metropolitan Kunitachi High School, where her brother's Thelonious Monk records pulled her away from the classical repertoire. She went on to Berklee College of Music in Boston and graduated in 1989, then moved to New York. Published biographies have her playing there in the bands of Jesse Davis and Greg Osby, toured the United States and Japan in 1991 with a quartet led by Joe Henderson, and worked with Betty Carter and Mingus Dynasty; those sideman stints are reported in biographies rather than itemized on record sleeves. One document that does survive is Ja-Hed, a 1991 Japanese-label album by Kenny Garrett, on which she plays piano beside Christopher Thomas on bass and Brian Blade on drums.

She came home in 1992, formed a trio with bassist Tomoyuki Shima and drummer Dairiki Hara, and recorded her debut, Wow, at Sound City in Tokyo on September 3, 4 and 5. Somethin' Else issued it in January 1993 and the trio record sold in numbers rare for a Japanese jazz debut, around 50,000 copies by one account. It won a Swing Journal Jazz Disc Award. Its eight tracks already showed her range: Duke Ellington's "Rockin' in Rhythm," Monk's "Brilliant Corners," Ornette Coleman's "Broadway Blues," and originals such as "The Jungular" and "B-Rush." She returned to the studio in April 1993 with bassist Rodney Whitaker and drummer Billy Higgins for Cruisin', which Blue Note released in the United States in 1994.

That spring she did what no Japanese bandleader had done: in May 1994 she led her own group for a week at the Village Vanguard in New York, with Reginald Veal on bass and Herlin Riley on drums, two members of Wynton Marsalis's working band. The trio was recorded there on May 6, 7 and 8, and the material came out as Live at the Village Vanguard (1994) and Live at the Village Vanguard II (1995). Japanese media treated her as a phenomenon, and in 1995 she received the Idemitsu Music Award. Piano Quintet Suite (1995) pushed in a more composed direction, with a five-piece band that included alto saxophonist Eiichi Hayashi and the Detroit trumpeter Marcus Belgrave.

Through 1996 and 1997 she worked with older American masters. In January 1996 she recorded Hat Trick in New York with alto saxophonist Jackie McLean, bassist Nat Reeves and drummer Lewis Nash. That summer her trio, now with bassist Shigeo Aramaki, played Montreux, Pori and Stuttgart, and the three concerts became Play, Piano, Play (1996) and a Montreux concert video. On September 29, 1996, she was the pianist on Joe Lovano's Tenor Time, recorded at Avatar Studios with Whitaker and Al Foster.

Fragile (1998) was a sharp turn. She played electric keyboards as well as piano, used Reginald Veal on electric bass, and drew on soul and rock sources: "You've Lost That Lovin' Feelin'," "Compared to What," "Hey Joe" and a "Sunshine of Your Love" jam, with Karriem Riggins, Motohiko Hino and Tamaya Honda sharing the drum chair. In January 1999 she recorded Cool Woods with saxophonist Phil Woods, bassist Ron Carter and drummer Bill Goodwin.

Then she stopped. Her mentor, the pianist Jaki Byard, died in 1999, and after a March 2000 concert in Osaka she went on a long hiatus. She resumed in 2005, played more regularly from 2007, and in July 2009 released Musical Moments, her first album as a leader in eleven years, dedicated to McLean, Henderson, Carter, Freddie Hubbard and the other jazz figures who gave her chances. She recorded Baroque for Verve in New York on March 24 to 29, 2010, with Veal, Whitaker, Riley and a front line of James Carter, Wycliffe Gordon and Nicholas Payton. Its centerpiece is a nineteen-minute treatment of "The Threepenny Opera," opening with an unaccompanied piano passage built on a score Byard left in her care.

In summer 2012 she announced her retirement from professional performance. On September 6, 2013, at the Saito Kinen Festival Matsumoto, she played Gershwin's Rhapsody in Blue with Seiji Ozawa conducting the Saito Kinen Orchestra, a one-night return that drew wide attention. She came back to the stage in September 2015 at Tokyo Jazz, and in June 2016 released Tea Times, an album with a large horn section, drummer Terreon Gully and bassist Yunior Terry, with Naruyoshi Kikuchi credited as producer.

Since then the work has multiplied. In November 2017 she issued two albums on the same day: Very Special, a program of standards and classical pieces, and Glamorous Life, a trio record of mostly original tunes with bassist Yosuke Inoue and drummer Shinnosuke Takahashi. Together they won the jazz prize at the 2018 CD Shop Awards. XII (2018) introduced her sextet with Akihiro Yoshimoto on tenor saxophone and flute, Miki Hirose on trumpet and Yuzo Kataoka on trombone, and she played Rhodes and clavinet beside piano. In 2019 she recorded Junko Onishi Presents Jatroit live at Blue Note Tokyo with bassist Robert Hurst and drummer Karriem Riggins. Shinjuku Pit Inn recordings from November 2019 became Live XI, the three-CD Unity All and the double LP The Most Live! in 2020, with the band expanded by guitarist Satoshi Yoshida and alto saxophonist David Negrete.

In 2021 she formed THE ORCHESTRA, a big band that grew out of the expanded sextet, and released Out of the Dawn in June, with a cover illustration credited to trumpeter Terumasa Hino. In September 2021 she recorded Grand Voyage with a quartet of Inoue, a drummer credited as 吉良創太 and percussionist Gen Ogimi; the novelist Haruki Murakami wrote its liner notes, and it was released on December 29, 2021.

The most recent record is American Classics, a solo piano album recorded at i-Plaza Hall in Tokyo on August 21, 2025, and released on March 4, 2026. Its program draws on pianists she admires and on songs made famous by singers such as Ella Fitzgerald and Billie Holiday, including "'Round Midnight" and a Duke Ellington medley, with "What a Wonderful World" closing it. Her official schedule lists concerts through the end of the year and into January 2027, including a Cotton Club date on January 9. She turned 59 this April.

Onishi names Duke Ellington, Thelonious Monk and Ornette Coleman as her central influences, and the repertoire on her first records bears that out: Wow includes Ellington, Monk and Coleman pieces, and Live at the Village Vanguard II has Monk's "Brilliant Corners" and a twenty-minute reading of the Japanese popular song "Ringo Oiwake." Reviewers have noted that she is less conservative in choosing repertoire than many of her contemporaries, taking on material by Coleman and Charles Mingus rather than recycling hard bop standards; the Mingus piece "Meditations for a Pair of Wire Cutters" appears on both Baroque and Jatroit.

Her playing sits in the post-bop mainstream, and the records are built on trio interplay, long forms ("Congeniality" runs 14 minutes on the first Vanguard album, "The Threepenny Opera" nearly 20 on Baroque) and a willingness to rearrange material from outside jazz. Piano Quintet Suite, Baroque, the Tea Times horn arrangements and the five-part "Suite Estaciones" on Out of the Dawn show a composer thinking in large forms, and the Baroque sessions reach into twentieth-century classical writing as well. Byard, her mentor and a pianist who was a favorite pianist of Mingus, is the other thread: Baroque and Jatroit both carry a Threepenny Opera piece dedicated to him. At the other pole she has recorded unaccompanied piano, from the two solo tracks on Baroque to the full solo program of American Classics, and she has repeatedly worked with electric keyboards and rock repertoire, most obviously on Fragile and on XII.

The Jazz.com encyclopedia notes that at the height of "Junko fever" in the mid-1990s she was marketed in Japan in a glamorized, pin-up style, while describing her as a serious pianist all the same.