Ronnie Foster was born on May 12, 1950, in Buffalo, New York, to Roslyn M. Foster. He was drawn to music at four and played his first paid job at fifteen, in a strip club. He started on piano, taught in the classical manner, and switched to organ after sitting in at a jam session that had one. He listened to Jimmy Smith, moved on to more adventurous organists such as Larry Young, took advice from the Buffalo organist Joe Madison, and practiced in a studio where a room with an organ rented for 60 cents an hour. He later recalled hearing Smith's Blue Note records on headphones at the Buffalo Public Library.

Foster worked Buffalo and New York clubs and built a following playing with musicians including Stanley Turrentine, Grant Green and George Benson. By the early 1970s he led a group called Energy II. His first recording as a sideman was Green's live album Alive!, taped on August 15, 1970, at the Cliche Lounge in Newark, New Jersey, where he plays organ on five of the tracks. Francis Wolff, the Blue Note co-founder who produced the date, took notice of him; Wolff died a few months later, and the label's Dr. George Butler signed Foster.

His debut as a leader, Two Headed Freap, was recorded at Rudy Van Gelder's studio in Englewood Cliffs, New Jersey, on January 20 and 21, 1972, with arrangements by Wade Marcus and a band that included guitarist Gene Bertoncini, bassist George Duvivier, harpist Gene Bianco, and Arthur Jenkins on congas. It mixed his own tunes, among them "Mystic Brew," with soul covers such as Al Green's "Let's Stay Together" and Gamble and Huff's "Drowning in the Sea of Love." Sweet Revival followed from sessions on December 14 and 15, 1972, produced and arranged by Horace Ott with Bernard Purdie on drums and a repertoire of soul hits including "Back Stabbers," "Me and Mrs. Jones," and "Superwoman." Blue Note recorded him with a trio of guitarist Gregory Miller and drummer Marvin Chappell at the Montreux Jazz Festival on July 5, 1973, for the live album in its Cookin' with Blue Note at Montreux series. For On the Avenue (1974, recorded at Electric Lady Studios in New York) he added clavinet, Moog synthesizer and vocals to the organ, with horn arrangements by Pee Wee Ellis and a production credit for Benson. Cheshire Cat, recorded at A&R Studios in March 1975 and also produced by Benson, who plays guitar on one track, was his last Blue Note album of the decade.

Foster then moved to Columbia, where Jerry Peters produced Love Satellite (1978) and Delight (1979). Delight brought in Benson, Stevie Wonder, the singer Phyllis Hyman, drummer Leon "Ndugu" Chancler and Robert "Pops" Popwell; its "We As Love" is the tune Benson recorded on his live Weekend in L.A. The Racer (1986), issued in Japan on Electric Bird and in the United States on ProJazz in 1987, was his last album under his own name for 36 years, as he concentrated on session and production work.

Benson was the center of that session work. Foster plays on his CTI album Good King Bad (recorded 1975), on Breezin' (1976), where he plays electric piano and Minimoog and wrote the tune "Lady," and on In Flight (1977), Weekend in L.A. (1978) and Livin' Inside Your Love (1979). He played organ on "Summer Soft" on Wonder's Songs in the Key of Life (1976), and he recorded with Roberta Flack, Smith (Unfinished Business, 1978), Lalo Schifrin, the Jacksons (Triumph, 1980) and Earl Klugh, whose Crazy for You (1981) brought Foster, Klugh and the arranger Clare Fischer a Grammy nomination for best arrangement on an instrumental recording. He was one of the keyboardists in Creed Taylor's Fuse One studio group, playing on its 1980 self-titled album and on Silk (1981). In 1987 he played keyboards on Turrentine's Wonderland, a set of Stevie Wonder songs for Blue Note, for which Discogs also credits him as producer and arranger, and he co-produced and wrote the opening track of Grover Washington Jr.'s Time Out of Mind (1989).

Production grew into a second career. Discogs credits him as producer on a long run of albums for the Brazilian singer Djavan beginning with Luz (1982), on records by Guilherme Arantes and João Bosco, and on several releases by the Puerto Rican pop singer Chayanne. He also produced Nestor Torres's Dance of the Phoenix (1991), most of the tracks on Will Downing and Gerald Albright's Pleasures of the Night (1998), which topped Billboard's contemporary jazz albums chart, and Roger Smith's Both Sides (1999), which reached number 24 among contemporary jazz albums.

Meanwhile his Blue Note records were found by a new generation. A Tribe Called Quest built "Electric Relaxation," from Midnight Marauders, on "Mystic Brew," and J. Cole later reworked the same track in "Forbidden Fruit" and, reversed, pitched and slowed, in "Neighbors." "Mystic Brew" has also been covered by the Vijay Iyer Trio and BadBadNotGood. Foster returned to Blue Note with Reboot, released on July 15, 2022, his first new album in 36 years, executive produced by label president Don Was and recorded over two days at Capitol Studios in Los Angeles. It features his son Chris Foster on drums, guitarist Michael O'Neill, and percussionists Luis Conte and Lenny Castro, includes a version of Stevie Wonder's "Isn't She Lovely," and is dedicated to the late Buffalo organist Lonnie Smith. His daughter Kaylie designed the cover. He toured Europe with the Reboot trio in the autumn of 2024.

Foster plays groove-based soul-jazz and jazz-funk organ, closer to the R&B and funk of his generation than to bebop, and his own albums lean on pop and soul repertoire, clavinet and Moog textures and tight, arranged horn parts. Blue Note presents him as the next in the label's line of Hammond B3 organists after Jimmy Smith, Larry Young and Lonnie Smith; at the time of Reboot he was playing a custom Hammond XK-5. His early models were Smith and Young, and in the studio he moved easily between organ, electric piano and synthesizers, which made him a regular keyboardist for George Benson, Earl Klugh and a run of pop-jazz sessions. Critics of the 1970s albums were divided: AllMusic called Two Headed Freap glitzy, cinematic funk that jazz purists might find monotonous and commercial, rated On the Avenue the most accomplished record of his career, and praised Cheshire Cat for balancing the soul and jazz sides of the equation. The same records became source material for acid jazz and hip-hop producers, and "Mystic Brew," through its sampling, became the track he is best known for.