He formed a band with schoolmate, John Osbourne(Ozzy Osbourne), called Rare Breed. They brokeup for a time, then reunited with Tony Iommi and Bill Ward. Their original name was Polka Tulk, then Earth, and eventually, Black Sabbath. He played rhythm guitar in this time, but Iommi made it clear to him that he was going to be the only guitarist, so Butler switched to bass.
He is noted as one of the first bassists to you a Wah-wah Pedal, and one of the firsts to "de-tune" his bass (In order to match Iommi when he tuned to a C#). Both of these are used in many metal bands today.
He is currently in Heaven and Hell, with other former Black Sabbath members, Tony Iommi Ronnie James Dio, and Vinny Appice.