Nicholls was originally brought in as a second guitarist when Black Sabbath doubted whether they would even continue under that name. Nicholls then switched to bass when Geezer Butler left briefly, and then became the band's keyboardist upon Butler's return and the decision to keep the Sabbath name. Nicholls' first appearance on a Black Sabbath album was on Heaven and Hell (1980), and he was credited as keyboardist on every Sabbath release from that time until 13 (2013), although he was not an official member until 1986. He remained an official member until 1991, then regained member status from 1993 to 1996. He was an unofficial member once again since the reunion with Ozzy Osbourne in 1997. Although his main role with Sabbath was on the keyboard, Nicholls also played some rhythm guitar on the reunion tours, e.g., during Iommi's solo in "Snowblind" and a few tracks during the Headless Cross (1989) and Forbidden (1995) tours.
Nicholls' involvement with the band ended when Adam Wakeman (a member of Ozzy Osbourne's solo band) was chosen to play keyboards during Sabbath's 2004 and 2005 tours as part of Ozzfest, and Scott Warren (Dio) handled keyboard duties on the 2007 Heaven & Hell tour.
Until his death, Nicholls played keyboards with former Black Sabbath singer Tony Martin, in his band Tony Martin's Headless Cross. Nicholls had previously performed on both of Martin's solo albums and their support tours.