Cowsill was the lead singer of the 1960s pop group The Cowsills, which was the inspiration for the family band depicted in The Partridge Family. The four Cowsill brothers formed the band, and then the mother and little sister joined the group later.

Cowsill suffered from emphysema and osteoporosis. Oddly, Cowsill's brother Barry disappeared during Hurricane Katrina, and his body was found four months later.