Ottawa firefighters were called to the historically designated mansion around 7:30 p.m. Tuesday when Commissioner Andrew Pocock and his wife noticed smoke, the Ottawa Sun reported.
Pocock said he called 911 and evacuated his family, staff and guests.
There were no injuries reported and damage estimates hadn't been tallied Wednesday.
Fire spokesman Marc Messier said the building has a steel roof, meaning firefighters had to fight the flames from inside, sawing into walls and ceilings to check for hot spots in the wood-frame building. The majority of the damage was done in the attic and Messier said crews did their best to protect heritage pieces and valuables.
The mansion known as Earnscliffe was the home of Canada's first prime minister, John A. MacDonald, who lived there from 1883 until he died there eight years later.