A Scottish keyboardist and co-founder of the Rolling Stones, often referred to as "The Sixth Stone". He was dismissed from the line-up in May 1963 but remained as road manager and piano player.

Born in Fife, Scotland, and raised in Sutton, Surrey, Stewart (often called Stu) started playing piano when he was 6. He took up banjo and played with amateur groups on both instruments. He loved R&B, boogie-woogie, blues and big-band jazz, and was first to respond to Brian Jones's advertisement in Jazz News in May 1962 seeking musicians to form a group. Mick Jagger and Keith Richards joined in June, and the group, with Dick Taylor on bass and Tony Chapman on drums, played their first gig under the name the Rollin' Stones at the Marquee Club in July 1962. By January 1963, Bill Wyman and Charlie Watts had joined, replacing a series of bassists and drummers.

Stewart also had a steady job with ICI and often handled the Stones' bookings at work. He also bought a van to transport the group and their equipment to gigs.

In May 1963, the band's manager, Andrew Loog Oldham, decided that Stewart should no longer be onstage, that 6 members were too many for a popular group and that Stewart didn't fit the image, but allowed Stewart to stay as road manager and play piano on recordings. Stewart accepted this demotion.

Stewart contributed piano, organ, marimbas and/or percussion to all of the Stones' albums released between 1964 and 1983, except for Beggars Banquet. He was not the only keyboard player who worked extensively with the band: Jack Nitzsche, Nicky Hopkins, Billy Preston and Ian McLagan all supplemented his work. In 1975, Stewart joined the band on stage again and played piano on tours in 1975-76, 1978 and 1981-82. He remained dedicated to boogie-woogie and early R&B.

Stewart contributed to Led Zeppelin's Rock and Roll on Led Zeppelin IV and Boogie With Stu on Physical Graffiti, both featuring his boogie-woogie style. He was also on Howlin' Wolf's 1971 London Sessions album, featuring Eric Clapton, Ringo Starr, Klaus Voormann, Steve Winwood, Bill Wyman and Charlie Watts, and played piano and organ on George Thorogood and the Destroyers' 1982 Bad To The Bone album.

Stewart also played with the back-to-roots band Rocket 88, a late-1970s/early-1980s venture which included Watts, Alexis Korner, Jack Bruce (on stand-up acoustic bass), Bob Hall (sharing piano with Stewart) and a horn and brass section including Colin Smith, John Picard, Dick Morrissey and Don Weller.

In December 1985, Stewart began having respiratory problems and died of a heart attack while waiting to have the problem examined.