Julius Arthur Hemphill was born January 24, 1938, in Fort Worth, Texas, and attended I.M. Terrell High School, the same segregated Fort Worth school that produced Ornette Coleman. He took up clarinet first, studying with the Fort Worth jazz clarinetist John Carter, another I.M. Terrell alumnus, before switching to saxophone; he played baritone in high school and cited Gerry Mulligan as an early influence. He went on to study music at North Texas State College. In 1964 Hemphill joined the United States Army and served several years in the U.S. Army Band; after his discharge he played for a time with Ike Turner's band before settling into St. Louis in 1968.

In St. Louis, Hemphill co-founded the Black Artists' Group (BAG), a multidisciplinary collective of musicians, actors, poets, dancers and visual artists that formed out of frustration with the limited opportunities available to Black artists in the city. Fellow saxophonists Oliver Lake and Hamiet Bluiett, trumpeters Baikida Carroll and Floyd LeFlore, and trombonist Joseph Bowie were among BAG's other core members. Around this time Hemphill started his own record label, Mbari, to document his music independently. Its first release, Dogon A.D., recorded in February 1972 at Archway Studios in St. Louis with Baikida Carroll on trumpet, Abdul Wadud on cello and Philip Wilson on drums, became one of the most acclaimed records of the free-jazz era and remains his best-known leader date. He followed it in 1977 with Blue Boyé, a solo album built from overdubbed alto, soprano and flute parts, on which a young Tim Berne, then studying saxophone with Hemphill, worked as assistant producer.

Hemphill moved to New York City in the mid-1970s and became active in the loft-jazz scene, recording as a sideman with Anthony Braxton and Lester Bowie and cutting his own Coon Bid'ness for Arista/Freedom in 1975 with Arthur Blythe, Hamiet Bluiett, Abdul Wadud and Barry Altschul. He had already been performing in all-saxophone ensembles with Braxton, and in 1976 he formalized the idea by founding the World Saxophone Quartet with Oliver Lake, Hamiet Bluiett and David Murray. The Quartet became the most durable and commercially successful of Hemphill's projects, recording steadily for Moers Music and Black Saint through the late 1970s and early 1980s (Point of No Return, 1977; Steppin' with the World Saxophone Quartet, 1979; W.S.Q., 1981; Revue, 1982) before moving to Nonesuch and Elektra later in the decade for Plays Duke Ellington (1986), Dances and Ballads (1987) and Rhythm and Blues (1989). Hemphill wrote much of the group's book and was widely regarded as its most distinctive composer. He also maintained a strong interest in theater, incorporating spoken narration into his 1977 solo album Roi Boyé & the Gotham Minstrels and later composing an extended theatrical work he called "a saxophone opera," Long Tongues.

Through the 1980s Hemphill kept up his own leader dates alongside the WSQ, including Flat-Out Jump Suite (1980, Black Saint) with Abdul Wadud, Olu Dara and Warren Smith, and Georgia Blue (1984, Minor Music), which featured a young Nels Cline on guitar. In 1988 he got his only opportunity to record his big-band writing on Julius Hemphill Big Band (Elektra Musician), a session that included Bill Frisell, Marty Ehrlich, John Purcell and J.D. Parran. He also composed the score for the Bill T. Jones/Arnie Zane Dance Company's Last Supper at Uncle Tom's Cabin/The Promised Land, performed by a Hemphill saxophone sextet, which premiered at the Brooklyn Academy of Music in November 1990 and earned Hemphill a Bessie Award (New York Dance and Performance Award) the following year.

Hemphill left the World Saxophone Quartet in the early 1990s and formed his own all-saxophone sextet with players including Marty Ehrlich, Andrew White and a young James Carter, debuting on Fat Man and the Hard Blues (1991, Black Saint). Worsening diabetes and subsequent heart surgery soon forced him to stop playing altogether; on the sextet's second album, Five Chord Stud (1993/94, Black Saint), Hemphill appears only as composer and conductor, with his former student Tim Berne taking the alto chair in his place. He remained a special guest on Björk's 1994 MTV Unplugged taping, later issued on her Debut Live release. Julius Hemphill passed away in New York City on April 2, 1995, at age 57, from complications of diabetes. He was inducted into the DownBeat Critics Poll Hall of Fame that same year.

Hemphill's sextet, subsequently led by Marty Ehrlich, continued performing and recording his compositions after his death, and interest in his catalog has only grown since: New World Records issued the seven-disc archival box set The Boyé Multi-National Crusade for Harmony, drawn from performances between 1977 and 2007, in 2021, and a previously unreleased 1982 duo recording with Ehrlich, Circle the Heart, appeared on Relative Pitch Records in January 2026. Hemphill recorded more than twenty albums as a leader and roughly ten with the World Saxophone Quartet over his career.

Hemphill's alto sound was famously huge and rough-edged, a tone critics have likened to a saxophone built from a length of steel pipe, and it carried a formidable technique in service of a restless compositional imagination that fused his jazz roots with elements of European classical music, African rhythm and American musical theater. He worked comfortably across a striking range of formats, from unaccompanied overdubbed solo records to duos, an all-saxophone sextet and, on his one chance to record for big band, a fourteen-piece ensemble, and he was widely credited as the World Saxophone Quartet's most individual writer even among three other strong composer-improvisers. He gave saxophone lessons to David Sanborn and Tim Berne, both of whom have cited his early Mbari recordings as formative, and his combination of a blues-and-gospel-rooted tone with structurally ambitious writing placed him alongside Anthony Braxton and the AACM generation as one of the defining composer-instrumentalists of the 1970s and '80s avant-garde.