Windsor Mayor Sylvie Bureau witnessed the early morning fire. She said she arrived shortly after 3 a.m., about 45 minutes after the fire started.
Bureau said everyone pitched in to help evacuate a nearby seniors' home as a precaution, in case the flames spread.
"There were plenty of volunteers who lent a hand," Bureau said.
She said 70 residents were helped to safety aboard heated buses to wait out the fire.
The church was frequented by many of the seniors.
"For them, it's an enormous shock," Bureau said. "It's a big loss."
Église St-Philippe, designated a Quebec heritage building, was started in 1892, completed at the turn of the 20th century and restored in 1980. It housed a 103-year-old Cassavant organ that was also destroyed in the fire.
Bureau said it was one of the few churches still open in the area, and it was a popular place of worship.
Officials have not yet determined the cause of the fire, but the mayor did confirm that there was major construction work being done outside the building.