Kamasi Tii Washington was born on February 18, 1981, in Los Angeles and grew up in the city's South Central district. His father, Rickey Washington, is a saxophonist and flutist, and the son took up his father's old saxophone at about thirteen. His first name comes from Kumasi, the Ghanaian city his father had visited shortly before he was born. He attended the Academy of Music at Alexander Hamilton High School and played in the L.A. Multi School Jazz Band, a program for gifted students run by the teacher Reggie Andrews, alongside future collaborators including the bassists Stephen "Thundercat" Bruner and Miles Mosley, the drummer Ronald Bruner Jr., the pianist Cameron Graves, the trombonist Ryan Porter and the saxophonist Terrace Martin. By his own account Andrews once pointed to him without warning and gave him a solo at the Playboy Jazz Festival, an experience that pushed him to practice about eight hours a day.

He then enrolled in the ethnomusicology department at UCLA, where he played with faculty members including Kenny Burrell, the bandleader Gerald Wilson and Billy Higgins. Higgins mentored a quartet of Washington, Graves and the Bruner brothers, which released Young Jazz Giants on Birdman Records in 2004. Washington joined the Gerald Wilson Orchestra for its 2005 album In My Time and appears on three more of its records through 2011; he has described Wilson as a mentor who encouraged him to write for strings. Between 2005 and 2008 he also put out three self-released albums with his own band, Live at 5th Street Dick's, The Proclamation and Light of the World, while the circle of young Los Angeles players performed at the 5th Street Dick's coffee shop and at the World Stage, the Leimert Park venue founded by Higgins. That circle became the West Coast Get Down, a collective formally established in 2009 at the Piano Bar in Hollywood with Mosley as its founder.

In December 2011 the collective rented Kingsize Soundlabs in Echo Park and recorded for thirty days, producing roughly 190 songs. Seventeen of them became The Epic (Brainfeeder, May 5, 2015), a three-disc, nearly three-hour album, his first on a label, released by the imprint of the producer Flying Lotus; his working band recorded it and he then added a string orchestra and choir that he arranged. Between those sessions and the release he had become one of the most visible sidemen in Los Angeles: he played on Snoop Dogg records, on George Duke's DreamWeaver (2013), on Harvey Mason's Chameleon and Stanley Clarke's Up (both 2014) and on Flying Lotus's You're Dead! (2014), and when Terrace Martin brought him into the sessions for Kendrick Lamar's To Pimp a Butterfly (March 2015) he contributed saxophone and string arrangements. The Epic reached number 3 on the US Top Jazz Albums chart and placed in the year-end top ten at both The Guardian and Pitchfork.

The follow-ups were larger in ambition rather than length. Harmony of Difference (Young Turks, September 29, 2017) is a six-movement suite for the full band that ends with a 13-minute finale, "Truth," which was used in the closing scene of the television drama Homeland. Heaven and Earth (June 22, 2018) is a two-disc, roughly three-hour set of originals and covers, including Freddie Hubbard's "Hub-Tones," packaged with a bonus disc, The Choice. In 2020 he scored Becoming, the Netflix documentary about Michelle Obama, earning a Primetime Emmy nomination and a Grammy nomination for the music. That year he also joined Robert Glasper, Terrace Martin and the producer 9th Wonder in the group Dinner Party, whose records blend jazz, soul and hip-hop; the group's Whatchu Bringing? followed on August 7, 2026.

Fearless Movement (Young, May 3, 2024), which he calls his "dance album," runs 86 minutes over twelve tracks and features André 3000, George Clinton and Thundercat; it was shaped in part by the birth of his daughter, who contributed to one song, and it reached number 1 on the UK Jazz & Blues Albums chart. In 2025 he composed the score for the anime series Lazarus, directed by Shinichiro Watanabe, with Bonobo and Floating Points, and he produced the debut album of his wife, the singer Fatima Zohra Washington, who records as Ami Taf Ra: The Prophet and the Madman (Brainfeeder, August 22, 2025). At the 68th Grammy Awards on February 1, 2026, he was among the credited songwriters on Kendrick Lamar and SZA's "luther" (Record of the Year) and on Lamar's "tv off" (Best Rap Song). His most recent release as of October 2026 is the Dinner Party album, and he remains active as a bandleader, collaborator and film and television composer.

Washington plays tenor saxophone in the modal and spiritual-jazz tradition of John Coltrane and Pharoah Sanders, and he writes for large forces rather than the usual small group: a rhythm section with two drummers and two bassists, horns, vocals, strings and choir. He names Coltrane's Transition, which his father first played for him when he was nine, as his biggest musical inspiration, and he credits Stravinsky's Symphony of Psalms as a model for the choir and strings on The Epic. One review of that album lists among its inspirations Pharoah Sanders's global-minded experiments, Horace Tapscott's Pan Afrikan Peoples Arkestra and the choir explorations of Donald Byrd and Eddie Gale. The grooves come as much from Los Angeles funk, soul and hip-hop as from bebop, a result of his years as a sideman for Snoop Dogg, Lauryn Hill, Raphael Saadiq, Chaka Khan and Kendrick Lamar. The Epic drew a Metacritic score of 83 and a place on Rolling Stone's list of the best albums of the 2010s, and it reached listeners who did not normally follow jazz. His credits in hip-hop, film and anime are a large part of his public profile, but his own records are jazz records, led, arranged and recorded with his jazz band.