Butler played rhythm guitar in his pre-Sabbath days, including with Rare Breed, but when Sabbath was formed, Iommi made it clear that he wouldn't want to play with another guitarist, so Butler moved to bass.
Butler is noted as being one of the first bassists to use a Wah-wah pedal on his bass, as showcased at the beginning of "N.I.B." which inspired many later bassists, such as Cliff Burton who cited him as an influence. He was also one of the first bassists to de-tune (from the standard EADG to the lower C#F#BE), in order to match Iommi who had started tuning his guitar to C# (one and a half steps down). De-tuning was soon adopted as a standard in many heavy metal bands to follow.
While Ozzy Osbourne was the focal point of the band, Butler wrote most of the band's lyrics, drawing heavily upon his fascination with the black arts to explore recurring themes of death and destruction.
During the latter half of the 1970s, Black Sabbath's popularity dwindled, although the group continued on in the early 1980s with ex-Rainbow frontman Ronnie James Dio and then with ex-Deep Purple frontman Ian Gillan. Butler quit the band in the middle of 1984, forming the Geezer Butler Band. In 1988 he joined Ozzy 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" lineup, but again quit the group after the Cross Purposes tour of 1994.
By 1995, Butler was back together 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 one more time 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 would be reuniting with Tony Iommi, Vinny Appice and Ronnie James Dio for a tour under the moniker Heaven and Hell - the latest incarnation of Black Sabbath.
He currently plays Lakland basses.