After the death of The Doors' lead singer Jim Morrison he managed the remaining three members during their recording sessions for two records. He was the one person in the United States Pamela Courson contacted from Paris immediately after Morrison had died. He arranged the funeral and burial with Pamela, but never saw Jim's body due to the closed casket and his aversion to seeing his friend dead. “We buried Jim correctly,” says Siddons, “and that perhaps was my greatest achievement: making sure we kept it quiet until it was done the right way. Nothing to hide, but we knew what was going to happen because we’d just been through it with Jimi and Janis."
Siddons began his career as a teenage roadie with the Doors while attending Cal State Long Beach, and six months later was asked to be the manager of the band. After Morrison's death, Siddons and the band parted ways when the band dissolved after recording two albums after Jim's death, reformed as "the Butts Band", and he suggested moving to a standard 15 per cent commission arrangement. They reunited around the release and marketing of the Jim Morrison poetry record, An American Prayer which relaunched awareness of The Doors, their historical significance and sextupled their royalties. His character appears in Oliver Stone's movie The Doors, in which Josh Evans plays him.
Siddons had this to say about his initial meeting with Jim Morrison: "So what was my first impression of Jim? He scared me to death."
Post-Doors, Bill Siddons continues his career as a manager in the music industry. He managed or co-managed such groups as Crosby, Stills, and Nash, Poco, America, Van Morrison, Pat Benatar, Jerry Cantrell, Robert Palmer, John Klemmer, and, in more recent times, was a co-founder of Core Entertainment, a professional management firm representing Alice in Chains, another band which dealt with the death of its frontman, and others. Siddons co-managed Alice in Chains with the band's original manager, Susan Silver, from 2005 to 2007. He currently runs Core Entertainment Org., and is partners with Jeremy Rosen Esq. in RoxCore Management, representing Ace of Base, Ozomatli, and Matisyahu.
He is married to comedian Elayne Boosler.