Cowsill, whose group inspired TV series The Partridge Family, had been ill for several months but no cause of death was given.
The singer, whose hits in the 60s and 70s included Hair, died in Calgary, Alberta, on Friday.
His brother and fellow band member Barry Cowsill drowned during Hurricane Katrina last year.
The news of William "Billy" Cowsill's death came during a memorial service for Barry.
Operation
"He is singing with the angels now," said Canadian record producer Neil MacGonigill, who first met William Cowsill in the 70s.
He had been suffering from emphysema, osteoporosis and other ailments and was recovering from back surgery, during which doctors had to collapse one of his lungs.
The band, which included four Cowsill brothers and later their mother and younger sister, was formed in Newport, Rhode Island in the early 1960s.
They got their big break when a producer for a US TV show booked them to appear, and they secured a record deal on the back of that performance.
The band broke up acrimoniously in the 1970s.
Cowsill, who moved to Canada 35 years ago, is survived by two sons, Travis and Del.
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©