Gary Lee Bartz was born September 26, 1940, in Baltimore, Maryland, the son of nightclub-owning parents whose venue brought touring jazz musicians through the house during his childhood. He picked up the alto saxophone at eleven, and as a teenager he got the chance to sit in with visiting players including Art Blakey and George Benson. After high school he enrolled at the Juilliard School in New York. From 1962 to 1964 he was a member of Charles Mingus's Jazz Workshop, where he worked alongside Eric Dolphy and first crossed paths with pianist McCoy Tyner. He also began gigging in the mid-1960s as a sideman with the Abbey Lincoln and Max Roach group, then briefly joined Art Blakey and the Jazz Messengers, making his recording debut on the band's 1965 album Soul Finger and remaining through 1966's Hold On, I'm Coming. In 1968 he joined Tyner's band Expansions, documented on the Blue Note album of the same name.

In 1968 Bartz formed his own band and signed to Milestone Records, debuting as a leader with Libra, a quintet date with drummer Billy Higgins, bassist Richard Davis, pianist Albert Dailey, and trumpeter Jimmy Owens. He followed it in 1969 with Another Earth, featuring Pharoah Sanders on tenor saxophone and Charles Tolliver on trumpet alongside bassist Reggie Workman, drummer Freddie Waits, and pianist Stanley Cowell; the album is now regarded as an influential early fusion of hard bop with modal and spiritual currents. In mid-1970, Miles Davis brought Bartz into his touring band on alto and soprano saxophone, a group that also included Keith Jarrett on electric keyboards, Michael Henderson on electric bass, Jack DeJohnette on drums, and, later, Airto Moreira on percussion and John McLaughlin on guitar. Bartz performed with Davis at the Isle of Wight Festival in August 1970 and at Washington, D.C.'s Cellar Door club that December; his playing from those engagements surfaced on Davis's 1971 album Live-Evil and, in full, on the 2005 box set The Cellar Door Sessions 1970.

Also in 1970, Bartz formed the NTU Troop, a group built around a fusion of jazz, funk, soul, and African and Caribbean musical elements, often with a spoken-word or vocal component. Vocalist Andy Bey joined the group for a run of Milestone and Prestige albums recorded between 1970 and 1974: Home! (a 1970 release drawn from a March 1969 concert at Baltimore's Left Bank Jazz Society, with Woody Shaw on trumpet), Harlem Bush Music: Taifa and Harlem Bush Music: Uhuru (both 1971, dedicated respectively to the memories of Malcolm X and John Coltrane), Juju Street Songs (1972), I've Known Rivers and Other Bodies (1973, recorded live at the Montreux Jazz Festival), Follow, the Medicine Man (1973), and the group's final outing, Singerella: A Ghetto Fairy Tale (1974), produced by Larry Mizell. That production relationship carried into 1975's The Shadow Do, made with Larry and Fonce Mizell alongside Davis's former bandmates Reggie Lucas, James Mtume, and Michael Henderson.

In 1977 Bartz again teamed with the Mizell brothers for Music Is My Sanctuary on Capitol Records, a jazz-funk album that reached the charts but which, in the words of AllMusic critic Thom Jurek, "initially proved far too funky for most jazz critics." He continued in a radio-friendly, pop-oriented direction for the rest of the decade with Love Affair (1978) and Bartz (1980, produced by Mtume and Lucas), then spent most of the 1980s working primarily as a sideman and billed collaborator, recording steadily with Tyner, Phyllis Hyman, Mtume, drummer Leon "Ndugu" Chancler, and Woody Shaw. He returned to hard-bop leader dates beginning with 1988's Monsoon and 1989's Reflections on Monk: The Final Frontier, followed by a run of live and studio albums for Candid and Atlantic: West 42nd Street (1990), There Goes the Neighborhood! (1991), Shadows (1992), The Red and Orange Poems (1994, praised in its liner notes by critic Stanley Crouch), and Episode 1: Children of Harlem (1994). In 1996 he assembled a larger studio band, including vocalist Jon Hendricks and pianist Cyrus Chestnut, for The Blues Chronicles: Tales of Life, and that same year recorded Spirit of '70 with organist Robert Walter and saxophonist Karl Denson. In 1998 Bartz won a Grammy Award for Best Latin Jazz Performance as a member of Roy Hargrove's Crisol on the album Habana.

Bartz joined the jazz studies faculty at Oberlin Conservatory of Music in 2001, where he holds the title of Professor of Jazz Saxophone. He released Soprano Stories in 2005, his first album-length showcase for his soprano playing, and later that year won a second Grammy, for Best Jazz Instrumental Album, as a sideman on McCoy Tyner's Illuminations, alongside Terence Blanchard, Christian McBride, and Lewis Nash. After a stretch of teaching and occasional performing, he returned as a bandleader with Coltrane Rules: Tao of a Music Warrior (2011) on his own OYO label. In 2015 he received the BNY Mellon Jazz Living Legacy Award at the Kennedy Center. In 2019, Revive Music brought him to New York's Winter Jazzfest to mark the 50th anniversary of Another Earth, a performance he reprised at the North Sea and Newport jazz festivals with original Another Earth personnel Charles Tolliver, plus drummer Nasheet Waits and saxophonist Ravi Coltrane. That same year, producer Gilles Peterson paired Bartz with the London spiritual-jazz group Maisha for the We Out Here festival; the collaboration led to 2020's Night Dreamer Direct-to-Disc Sessions, reworking earlier NTU Troop material. Bartz went on to record two volumes in Adrian Younge and Ali Shaheed Muhammad's Jazz Is Dead series. In 2024, the National Endowment for the Arts named him an NEA Jazz Master. He formed a new lineup of NTU and released The Eternal Tenure of Sound: Damage Control in 2025, and in May 2026 received an honorary Doctor of Music degree from the New England Conservatory.

Bartz's alto style is grounded in bebop and hard-bop vocabulary but carries an unmistakable modal and spiritual cast absorbed from his time around John Coltrane's generation, delivered with a vocal, cutting tone on both alto and soprano. What sets his catalog apart is the range: he moved between acoustic post-bop leader dates, the genre-blending jazz-funk-soul-African fusion of NTU Troop, and a run of unabashedly commercial 1970s crossover records, then returned to hard bop in the 1990s without ever fully leaving any of those lanes behind. That range, and NTU Troop's fusion of jazz with funk, soul, and spoken word in particular, has been cited as a direct influence on later spiritual-jazz and fusion-minded musicians including Kamasi Washington, Yazz Ahmed, and Shabaka Hutchings. Critic Stanley Crouch, writing in the liner notes to The Red and Orange Poems, called him "one of the very finest to have ever picked up the instrument."