Butler received the nickname "Geezer" at approximately age eight, because he "used to call everybody Geezer" at school. "It was just a slang term for a man." 
Butler grew up in a working-class Irish Catholic family and was heavily influenced by the writing of Aleister Crowley as a teenager. Butler formed his first band, Rare Breed, in the autumn of 1967, with John "Ozzy" Osbourne soon joining as lead vocalist. Butler dated a girl who lived near Tony Iommi, and Iommi's earliest memories of Butler involved seeing him walking past his house in Birmingham quite often to visit her. Later, Iommi and Butler became acquainted when their bands played at a nearby nightclub. Separated for a time, Osbourne and Butler reunited in the blues foursome, Polka Tulk, along with guitarist Iommi and drummer Bill Ward. They renamed their band Earth, but after finding a band in the small-time English circuit with the same name, soon adopted Black Sabbath in early 1969.
Inspired by John Lennon, Butler played rhythm guitar in his pre-Sabbath days, including with Rare Breed. When Sabbath was formed, Iommi made it clear that he did not want to play with another guitarist, so Butler moved to bass. Butler lists Jack Bruce of Cream as his biggest influence as a bassist. Iommi described Butler as being "from another planet" in the band's early days; he took LSD, wore Indian hippie dresses, and was very peaceful. At the time Black Sabbath was formed, Butler was studying to become an accountant, and this training resulted in him managing the band's finances in the early days.
Butler briefly left Black Sabbath during the recording of their 1980 album Heaven and Hell to deal with personal problems. He again left the band in 1984 after touring in support of their 1983 album, Born Again. In 1988 he joined his former Sabbath bandmate Osbourne to take part in the No Rest for the Wicked World Tour. Butler re-joined Black Sabbath in 1991 for the reunion of the Mob Rules line-up, but again quit the group after the Cross Purposes tour in 1994.
In 1995 Butler joined with Osbourne to play on the Ozzmosis album. After recording Ozzmosis, he formed G/Z/R, issuing Plastic Planet in 1995. His next solo album, Black Science, followed in 1997. Butler returned to Sabbath once more for the 1997 edition of Ozzfest, and has remained with the band since. In 2005 he released Ohmwork, his third solo album. In October 2006 it was announced that Butler, along with Tony Iommi, would be reforming the Dehumanizer-era Black Sabbath line-up with Vinny Appice and Ronnie James Dio, under the name Heaven & Hell to differentiate between the reunited touring band fronted by Osbourne, and the current Sabbath line-up. He recorded and toured for the album "13" (2013) and the subsequent tour, reaching an end for the final tour in 2017.