The incident happened at the Dundas Cactus Festival outside Hamilton.
Witnesses said McMillan, who had complained of chest pains throughout the day, was standing near the back of the stage holding his bass guitar when he collapsed.
Rob Violette, owner of First Impressions, a Sudbury-based entertainment broker company, said he had worked for many years with McMillan.
"I was only a business acquaintance, but I got to know him pretty good," said Violette. "He had worked for many, many years in the business and he was a very well-respected guy and very talented bass player and musician.
"He was very ambitious and always playing. He was very polite and professional. He's going to be missed."
McMillan had played in two different Shania Twain cover bands for the past couple of years and spent several years in other bands that played tributes to established music superstars, said Violette.
"He was considered one of the best bass players around."
Emergency personnel arrived within minutes of McMillan collapsing, but they were not able to revive him. He was pronounced dead at hospital.
McMillan is survived by his wife Shelley Butler of Garson, three children and a brother.
