York Regional Police say the incident happened just before noon Tuesday at Bethesda Grange Golf Course in Whitchurch-Stouffville.
Witnesses at the golf course say they heard a horn warning them to get off the course and shortly afterwards heard a loud crack, believed to be the lightning.
Some of those at the scene rushed to assist the injured golfers by giving them first aid and CPR.
Peter Epstein, one of the first people to reach the injured, says one of the men had no vital signs but was revived by an ICU nurse who happened to be playing golf, and a firefighter who arrived.
He says the man, who he described as "dead" when found, had many burns and melted clothing.
"His hat was just melted, it was just completely blown off his head," he said.
He said the golfers who were hit appeared to be in the same group, about 100 metres from the clubhouse.
"We heard the horn ... and then the next thing you know, just this big flash and bang, all at the same time," said Epstein, who was at the golf course for a tournament. "It was wild."
Environment Canada had earlier issued a severe thunderstorm watch for southern Ontario, including Toronto and the Greater Toronto Area.