The Bucks said the 86-year-old Fitzgerald died Monday morning, but a cause of death was not released. Schneider Apfel Schneider & Schneider funeral home said Fitzgerald died in his hometown of Janesville.
"Jim Fitzgerald was the right man at the right time for the Bucks franchise," said team vice president of business operations John Steinmiller. "He will be dearly missed by the entire Bucks family."
Fitzgerald joined the Bucks' board of directors in 1973, then became president and chairman of the board in 1976. He was owner during a successful era that included the hiring of Hall of Fame coach Don Nelson and a string of consecutive division titles from 1979-1980 to 1985-1986.
He sold the team to current owner Herb Kohl in 1985.
Fitzgerald was also part of a business team that owned the Warriors. He and a partner sold their majority ownership in 1995.
Fitzgerald, known as "Fitz," was a Navy veteran served during the Korean conflict. He returned to Janesville, where he opened several businesses.
"Jim was a great man and he taught us more than just basketball," former Bucks player Junior Bridgeman said in a statement. He said a group got together to visit Fitzgerald last year, something he said players don't usually do unless there's an "affinity" that goes beyond just us playing for someone.
"It was a family, and today we lost a member of that family," he said.
Fitzgerald is survived by his wife, six children, 15 grandchildren and nine great-grandchildren.
Funeral services will be held Thursday morning at St. John Vianney Catholic Church in Janesville, with burial at Mt. Olivet Cemetery.