Unser triumphed despite the effects of a severe case of food poisoning that had kept him in bed all day on Friday and left him in a weakened condition for the race. Robby Gordon was second.
Mansell, the defending Indycar champion, tried to pass Unser's Penske team- mate Emerson Fittipaldi for fourth place, but Fittipaldi, who apparently did not see Mansell on his inside, slammed the Englishman's car into a concrete barrier.
The angry Fittipaldi finished ninth, but remained second behind Unser in the championship standings. 'I'm very disappointed a champion like Nigel would do something like that,' Fittipaldi said. 'He tried to stop the car with my brakes. Nigel tried to pass me in an impossible place.'
Mansell had a different view. 'I went down the inside and he had the choice of letting me go or crashing, so we crashed,' he said. The Newman-Haas driver, who was berated by Fittipaldi after emerging from his car, finished the chaotic 102-lap race in 10th place after a solid run during which he led from lap 37 through to 76.
Michael Andretti had the fastest car on the track, but was slowed by two punctures and troublesome rear brakes. Andretti finished third, but not before he locked up and bumped Paul Tracy, the third Penske driver, out of the race.
The race was watched by 70,639 spectators - the biggest one-day attendance in Canadian sporting history.