Cowsill, who was suffering from emphysema, osteoporosis, and other ailments, died in Calgary, Alberta, on Friday, according to the family and Canadian record producer Neil MacGonigill. He had been in deteriorating health.
The Cowsills, inspiration for the "The Partridge Family," recorded a series of top hits between 1967 and 1970, including "The Rain, The Park and Other Things" and "Hair."
Four Cowsill brothers played in the band: Barry on bass, William on guitar, Bob on guitar and organ, and John on drums. Their mother, Barbara, and little sister, Susan, eventually joined the group.
Barry disappeared after Hurricane Katrina struck New Orleans on Aug. 29. His body was recovered Dec. 28 from the Chartres Street Wharf.
The band's career began in Newport, R.I. They were spotted by a producer for NBC's "Today" show which booked them for an appearance that led to a record deal.
The band broke up in the 1970s. William, the oldest brother, moved to Canada about 35 years ago, where he continued his music career with Blue Northern, The Blue Shadows and the Co-Dependents.
Cowsill is survived by two sons.