"He is singing with the angels now," said Canadian record producer Neil MacGonigill, who first met the singer with the pure, soaring voice in the 70s.
The Cowsills, the inspiration for the TV series The Partridge Family, recorded a series of top hits between 1967 and 1970, including The Rain, The Park and Other Things and Hair.
At one time the band members were also were spokespeople for the American Dairy Association, appearing in commercials and print ads for milk.
Cowsill, who was suffering from emphysema, osteoporosis, Cushing syndrome and other ailments, died in Calgary, Alberta, on Friday.
He had been in deteriorating health for a number of months. He was also still coping with the aftereffects of an eight-hour back surgery during which one lung had to be collapsed.
Doctors could not get it to function again but no cause of death has been released.
Four Cowsill brothers played in the band: Barry on bass, Bill on guitar, Bob on guitar and organ, and John on drums. Their mother, Barbara, and little sister, Susan, eventually joined the group.
The band's career began in Newport, Rhode Island, where by 1965 they had a regular gig at a club. They were spotted by a producer for NBC's Today show who booked them for an appearance that led to a record deal.
The band had an acrimonious breakup in the 1970s. William, the oldest member, moved to Canada about 35 years ago.
The news of Cowsill's death came during a memorial service being held at Newport's King Park honouring his brother Barry, who drowned after Hurricane Katrina.
William Cowsill is survived by two sons, Travis and Del.