The Republican-American of Waterbury reported on its Web site Wednesday that Mahan suffered from a brain tumor and had been hospitalized since Dec. 26.
"He loved the thought of being involved in a sport that was involved with glorious animals," Knowlton said. "It was his passion."
Mahan owned a 20 percent interest in Funny Cide, who became the first New York-bred horse to win the Kentucky Derby -- and the first gelding since Clyde Van Dusen in 1929.
Mahan was one of a group of horse racing novices that included retirees, teachers, an optician and a health care consultant who invested in Sackatoga Stable in 1995.
The group paid $17,500 for its first horse and won about $100,000. In 1999, the group hired trainer Barclay Tagg, who had never reached the Derby, and Tagg found Funny Cide for the bargain price of $75,000.
Funny Cide captured the Preakness by nearly 10 lengths and finished third in the Belmont Stakes.
Mahan and his partners became known at Churchill Downs for arriving in a large yellow school bus while other stable owners were driving luxury cars.