John Arthur "Jaki" Byard was born June 15, 1922, in Worcester, Massachusetts, to musician parents, John Sr. and Geraldine Garr Byard. He began piano lessons at six but the Depression cut them short, and much of his early education came from listening to bands at Lake Quinsigamond and to Fats Waller and Benny Goodman on the radio; he was working professionally on piano with local bands by sixteen. Drafted into the U.S. Army in 1941, he studied piano with Ernie Washington and took up trombone while stationed in Florida, where he became friendly with and mentored a teenaged alto saxophonist from nearby Tampa, Cannonball Adderley. Discharged in 1946, Byard settled into Boston's jazz scene for the next decade and a half: he toured roughly a year with Earl Bostic's band starting in 1947, led his own bebop group with trumpeter Joe Gordon and saxophonist Sam Rivers, spent three years working with altoist Charlie Mariano in Lynn, Massachusetts, and played tenor saxophone in Herb Pomeroy's Boston big band from 1952 to 1955. From 1959 to 1962 he toured with Maynard Ferguson's orchestra, an arrangement he later said constrained his taste for odd meters and freer improvising. During this stretch he cut his first records as a leader for Prestige's New Jazz subsidiary, the trio dates Here's Jaki (recorded March 1961) and Hi-Fly (recorded January 1962), both with bassist Ron Carter.

Byard's most consequential association began when he joined bassist and composer Charles Mingus's working band in the early 1960s. On October 12, 1962, he was one of two pianists, alongside guest Toshiko Akiyoshi, that Mingus used to cover his own sprawling charts at a notorious Town Hall concert in New York. Conceived by Mingus as an open working session and by producer George Wein as a finished concert, the date was chronically underrehearsed and interrupted, and it premiered sections of what later became the extended suite "Epitaph"; despite the chaos, it stands as one of the more storied nights in the music's history, and it is where Byard and Akiyoshi's names first appear together on a bandstand.

Byard remained Mingus's regular pianist through the mid-1960s, appearing on the dense orchestral suite The Black Saint and the Sinner Lady (Impulse!, 1963) and on Mingus Mingus Mingus Mingus Mingus (Impulse!, 1964). In the spring of 1964 he toured Europe in Mingus's classic sextet alongside multi-reedist Eric Dolphy, tenor saxophonist Clifford Jordan, trumpeter Johnny Coles, and drummer Dannie Richmond, a band documented on the February 4, 1964 Town Hall Concert recording and on various European concert tapes; it was Dolphy's last tour with Mingus before his death in Berlin that June. Byard rejoined Mingus for European dates again in 1970 and, after Mingus's 1979 death, played in the repertory Mingus Big Band organized by Sue Mingus, recording with that group by the late 1980s. The same Prestige/Mingus circle produced several of Byard's other important sideman dates of the period: Eric Dolphy's leader debut Outward Bound (New Jazz, 1960) and Far Cry (New Jazz, 1962, opposite trumpeter Booker Little), tenor saxophonist Booker Ervin's The Freedom Book (Prestige, 1964), and multi-instrumentalist Roland Kirk's Rip, Rig and Panic (Limelight, 1965) with Elvin Jones on drums.

Byard built a parallel run of his own Prestige leader dates across the same years. Out Front! (recorded 1964, released 1965), a quintet with trumpeter Richard Williams and tenor saxophonist Booker Ervin, is widely regarded as one of his finest records as a leader. Freedom Together! (1966) introduced the working trio of bassist/cellist Richard Davis and drummer Alan Dawson that critic Gary Giddins later called one of the most commanding rhythm sections of the decade; the same partnership, expanded with guitarist George Benson and Ellington violinist Ray Nance, cut Jaki Byard with Strings! (1968). Solo Piano (recorded 1969) distilled his full stylistic range, from stride and boogie-woogie through bebop to free playing, into a single unaccompanied recital. An earlier, previously unissued solo/trio session Byard recorded for critic Nat Hentoff in December 1960 finally surfaced in 1971 as Blues for Smoke (Candid), and the trio date Sunshine of My Soul (recorded October 31, 1967, Prestige), with bassist David Izenzon and Elvin Jones on drums and timpani, remains one of his more overlooked period records.

From 1969, Byard was a charter member of the jazz studies faculty at the New England Conservatory, teaching there for more than fifteen years and later also at the Hartt School, Manhattan School of Music, the New School, and Harvard University; pianist Jason Moran, who studied with him for four years, and Fred Hersch, who described him as "both organized and chaotic as a teacher," are among the students who credit him directly. In the late 1970s he fronted two editions of a big band he called the Apollo Stompers, one of working New York musicians and one of New England Conservatory students, documented on Phantasies (Soul Note, recorded 1984, released 1985) and Phantasies II (1991). He held a 1974-75 residency at Bradley's in New York, occasionally sat in with Duke Ellington's orchestra in 1974, and recorded one of his most fully realized later leader albums, Family Man (Muse, recorded 1978, released 1979), alongside bassist/tubist Major Holley and drummer/percussionist Warren Smith. A February 1972 duo session with pianist Earl Hines, one of the music's founding stride and swing voices, was issued in 1975 as Duet! (MPS/BASF) and paired Byard with a direct predecessor in the same eclectic, all-of-jazz-history lineage. He continued recording steadily into the 1980s and 1990s for Soul Note, Concord, Cadence, and other independent labels, including the two-piano session Improvisations (1982) with Ran Blake and the solo recital Live at Maybeck Recital Hall, Volume Seventeen (Concord, 1992).

Byard died on February 11, 1999, shot once in the head at his home in the Hollis section of Queens, New York, at age 76. Police found no signs of forced entry, struggle, or robbery, and no weapon was recovered; the killing was ruled a homicide and the case remains unsolved. He was survived by two daughters, a son, and several grandchildren; his wife of four decades had died five years earlier. He is credited with at least 35 albums as a leader and more than 50 as a sideman, and after his death, saxophonist Jamie Baum organized a tribute ensemble, Yard Byard, built around his unpublished compositions.

Byard was, by wide critical agreement, jazz piano's most complete stylistic chameleon: a single solo could move from ragtime and stride through Art Tatum-style arpeggios, Fats Waller stride, Thelonious Monk's angular phrasing, bebop, and Cecil Taylor-style dissonance, without ever feeling like pastiche. Critic Gary Giddins praised his "unfailingly bright" tone and his ability to float "barely articulated keyboard washes" beyond the harmonic bounds while staying anchored in the blues, and critic John S. Wilson described him progressing within a single solo "from a basic melodic statement to nimble Art Tatum fingering to Fats Waller stride, to prickly Thelonious Monk phrases, to Cecil Taylor dissonances." Byard himself insisted the humor in his playing did not undercut its seriousness. Underappreciated commercially in his own lifetime, he became a defining influence on musicians who treat the entire history of jazz piano as usable material, most visibly his New England Conservatory student Jason Moran, and his years anchoring Charles Mingus's bands made him one of the most important pianists in that leader's book.