Gil Evans was born Gilmore Ian Ernest Green on May 13, 1912, in Toronto, Canada. His biological father, reportedly a doctor, died before he was born; his mother, Margaret Julia McConnachy, later remarried a miner named John Evans, whose surname the boy took. The family moved constantly through Evans's childhood, chasing his stepfather's work through Saskatchewan, British Columbia, Washington, Idaho, Montana, and Oregon before finally settling in California, where Evans graduated from Stockton High School in 1930. He discovered jazz through radio and records as a teenager, drawn especially to Duke Ellington, Louis Armstrong, and Fletcher Henderson — models whose orchestral thinking would shape his own arranging for the rest of his life.

Entirely self-taught as a musician, Evans enrolled at the College of the Pacific in Stockton and in 1933 founded his own band, Gil Evans & His Orchestra. By 1935 the group was the house band at the Rendezvous Ballroom in Balboa Beach, California; it toured the Pacific Northwest in 1937 before relocating to Hollywood, where it became a regular fixture on Bob Hope's radio program. Even in these early charts, Evans was already reaching past a standard dance-band palette toward the French horns, flutes, and tubas that would become his signature.

From 1941 to 1948, Evans arranged for the Claude Thornhill Orchestra, a band whose lush, almost classical sound became the single largest influence on his mature style. His charts were notoriously difficult — bassist Bill Crow recalled that Thornhill would pull out Evans's toughest arrangements "when he wanted to punish the band" — but they were also where Evans first built the augmented brass instrumentation, standard big-band lineup plus French horn and tuba, that defined his orchestral voice. His basement apartment behind a Manhattan Chinese laundry became an informal salon during these years, a gathering place for musicians looking for an alternative to bebop's harmonic language; Charlie Parker, Gerry Mulligan, and John Carisi were regulars, and it was in these conversations that the ideas behind Birth of the Cool first took shape.

That thinking became concrete in 1948, when Evans, Miles Davis, Gerry Mulligan, and others assembled a nine-piece band built around the unusual voicings Evans had developed under Thornhill. The Miles Davis Nonet recorded twelve sides across three sessions for Capitol in 1949 and 1950; compiled and released in 1957 as Birth of the Cool, that material became one of the foundational documents of cool and West Coast jazz, and cemented Evans and Davis as creative partners.

Nearly a decade later, producer George Avakian gave Davis his pick of arrangers for a new Columbia project, and Davis chose Evans without hesitation, opening the most celebrated collaboration of either man's career. Miles Ahead (1957) established the template: Davis's trumpet and flugelhorn floating over an orchestra Evans built and conducted, credited on the sleeve as "Miles Davis + 19" with Evans directing. Porgy and Bess (1958) followed with dense, demanding arrangements of Gershwin's opera, completed under such tight rehearsal time that some errors made it onto tape — yet the record is still regarded as among the finest jazz treatments of that score in any idiom. Sketches of Spain (1960), built around Evans's love of Manuel de Falla and Joaquín Rodrigo, won the pair a Grammy for Best Jazz Composition of More Than Five Minutes Duration and remains the high-water mark of orchestral jazz. A fourth and more contentious record, Quiet Nights (1963), was released by Columbia against both Davis's and Evans's wishes, using a partial batch of unfinished bossa nova sessions; it strained Davis's relationship with producer Teo Macero for two years afterward. Evans also contributed uncredited arranging work to Davis's Filles de Kilimanjaro (1968), a late, quiet coda to their partnership.

Evans built a leader career in parallel with the Davis records rather than after them. Gil Evans & Ten (Prestige, 1957) was his recording debut as a bandleader, a small-group session featuring Steve Lacy on soprano saxophone. New Bottle Old Wine (World Pacific, 1958) paired his orchestra with a young Cannonball Adderley on reinterpretations of classic jazz repertoire, and Great Jazz Standards (World Pacific, 1959) continued in that vein with trumpeter Johnny Coles as featured soloist. Out of the Cool (Impulse!, 1961) is widely considered his own masterpiece as a leader: a Grammy-nominated set built around slow tempos, spacious voicings, and a front line including Coles, Ron Carter, and Elvin Jones, it's frequently cited alongside the Davis records as proof of how far Evans could push an orchestra without losing intimacy. Into the Hot (Impulse!, 1962) split billing between charts by John Carisi and a quintet led by Cecil Taylor, with Evans credited as producer/leader of the overall project rather than composer of every track on it — a credit worth noting for anyone assembling his discography from the spine alone. The Individualism of Gil Evans (Verve, 1964) pushed furthest into his mature orchestral language: five French horns, multiple bassists playing simultaneously (some arco, some pizzicato — Richard Davis, Paul Chambers, Ron Carter, Gary Peacock, Milt Hinton, and Ben Tucker all appear across the sessions), alto and bass flutes, oboe, and harp, with Eric Dolphy, Wayne Shorter, and Steve Lacy among the reed soloists. Its orchestral arrangement of Willie Dixon's "Spoonful" foreshadowed the genre-crossing Evans would return to a decade later.

The mid-1960s were harder going. Evans arranged the big-band tracks on Kenny Burrell's Guitar Forms (1965) and recorded Look to the Rainbow (1966) with Astrud Gilberto, but grew discouraged with Verve's push toward more commercial material, and his output slowed through a period marked by depression and frustration with the logistics of scoring for the large, specialized ensembles his music required.

The turnaround came from an unlikely source: his wife encouraged him to listen to Jimi Hendrix, and the encounter reignited him. Evans built a new, electric-instrumentation band — guitars, electric bass, synthesizers, no more French-horn choir — and spent the 1970s reinventing his sound around it. Gil Evans (Ampex, 1970, later reissued as Blues in Orbit) and Svengali (Atlantic, 1973, a live recording and Grammy nominee whose title is an anagram of his own name) opened the decade; The Gil Evans Orchestra Plays the Music of Jimi Hendrix (RCA Victor, 1974), arranged with guitarists John Abercrombie and Ryo Kawasaki in the band, made the Hendrix debt explicit. There Comes a Time (RCA, 1976) and the belatedly released Where Flamingos Fly (recorded 1971, issued on Artists House in 1981) carried the same electric-orchestral language forward, and Priestess (recorded live in 1977, released on Antilles in 1983) earned Evans another Grammy nomination.

In April 1983, at the invitation of club owner and producer Horst Liepolt, Evans's orchestra began a regular Monday-night residency at Sweet Basil in Greenwich Village that ran for nearly five years. The Monday Night Orchestra drew heavily from New York's session-player pool — many of its members, including saxophonist David Sanborn, trombonist Tom Malone, and trumpeters Lew Soloff and Alan Rubin, also played in the Saturday Night Live band — and the shifting, rotating lineup gave Evans's late book a constant supply of new voices, including his own son Miles Evans on trumpet. Bud and Bird, recorded live at Sweet Basil in December 1986 and released in 1987, won Evans a Grammy for Best Jazz Instrumental Performance, Big Band, in 1988.

Evans's final years brought a wave of outside work: he produced and arranged the soundtrack to the film Absolute Beginners (1986), working with Sade, the Style Council, Jerry Dammers, and David Bowie, and arranged music for Martin Scorsese's The Color of Money (1986). In July 1987 he led the Monday Night Orchestra plus saxophonist Branford Marsalis through a live set of Police songs and Hendrix material with Sting at the Perugia Jazz Festival — the Last Session recording — a project notable for introducing a young apprentice arranger, Maria Schneider, to Evans's circle. His final project was a set of arrangements written for the Laurent Cugny Big Band in Paris, recorded in November 1987 and released posthumously as Golden Hair (EmArcy, 1989).

Evans married Lillian Grace not long after the Birth of the Cool sessions, and married his second wife, Anita Cooper, in 1963; the couple had two sons, Noah and Miles, with Miles Evans eventually playing trumpet in his father's own orchestra. Gil Evans died on March 20, 1988, in Cuernavaca, Mexico, of peritonitis following prostate surgery, at age 75. He was inducted into the DownBeat Jazz Hall of Fame in 1986 and the Canadian Music Hall of Fame in 1997, and Miles Davis & Gil Evans: The Complete Columbia Studio Recordings won a posthumous Grammy for Best Historical Album in 1997. Conductor Ryan Truesdell's Gil Evans Project, authorized by the Evans estate, released Centennial in 2012 — a collection of previously unrecorded Evans compositions and arrangements that earned its own Grammy nomination and prompted Miles Evans to lead a centennial tribute concert of his father's music at New York's Highline Ballroom.

Evans's orchestral language grew directly out of his years arranging for Claude Thornhill: unusual brass and reed colorations, especially French horn and tuba layered into a standard big-band frame, replacing the sharp attack of conventional swing writing with something closer to a wash of sustained, blended color. Where most big-band arrangers wrote for sections, Evans increasingly wrote for individual timbres — a single flute doubling a bass clarinet, two bassists playing the same line arco and pizzicato at once — building material specifically around the soloists in front of him (Steve Lacy, Johnny Coles, Eric Dolphy, Wayne Shorter, David Sanborn) rather than treating personnel as interchangeable. That instinct made him, alongside his Davis collaborations, one of the most consequential orchestrators jazz has produced: a player of harmonic and textural possibility as much as a conventional bandleader. The other defining thread of his career was restlessness. He moved from the classically tinged big-band writing of the 1940s and '50s, through the operatic scale of the Davis Columbia albums, into the free-leaning, soloist-driven records of the early-to-mid 1960s, and finally into an electric, rock- and funk-inflected idiom in the 1970s and '80s built around Jimi Hendrix's music — a reinvention few arrangers his age attempted, let alone pulled off with a working band of New York's best session players.