Hydro One says at the height of the storm the power was knocked out to well over 100,000 homes and businesses between Windsor and the Kingston area.
In the Greater Toronto Area some 70,000 customers were left in the dark, and another 24,000 in London, but crews worked through the night to get the lights back on.
As of 7 a.m. Monday, several thousand customers remained without power but the utility expected to have most back up by later in the day.
The weather system roared into Ontario after punishing the American Midwest with tornadoes and thunderstorms that left at least six dead in Illinois.
In the central Illinois town of Washington, a twister obliterated entire neighbourhoods, flipping vehicles, uprooting trees, and ripping down power lines.
The storms also caused damage in Kentucky, Wisconsin, Indiana, Ohio and Michigan.