The McLaren-Honda domination continued in qualifying in Montréal but on this occasion it was Alain Prost ahead of Ayrton Senna. Riccardo Patrese was third quickest in his Williams-Renault with Gerhard Berger fourth for Ferrari, ahead of his team mate Nigel Mansell and Thierry Boutsen in the second Williams. Stefano Modena (Brabham) and Alex Caffi (Scuderia Italia) shared the fourth row with Andrea de Cesaris next in his Dallara ahead of Philippe Alliot's Larrousse Lola. The Benetton team was struggling with Sandro Nannini 13th and Johnny Herbert failing to qualify - and a power struggle developing inside the team between Peter Collins and Flavio Briatore.

It was raining on race morning and the starting procedure was a mess as Mansell, Nannini and Luis Pérez-Sala (Minardi) decided to pit at the end of the parade lap to change on to slick tyres. This was done and both the Ferrari and the Benetton went out of the pitlane at high speed, thinking that the race had already begun. In fact the others had not started and so in the early laps Mansell and Nannini ran at the front. They were both black-flagged amid recriminations that there should have been some official guidance at the pit exit.

Behind them Prost had gone into the lead at the start with Senna second and Patrese third ahead of Berger, Boutsen and de Cesaris. Caffi and Prost both pitted for slicks at the end of the first lap but soon afterwards Prost went out with a suspension failure on his McLaren. The order was jumbled as others pitted for slicks and the result was that Patrese was in the lead from Boutsen. Stefan Johansson provided an unusual spectacle when he rejoined after his stop with his Onyx trailing an tyre hammer, an air line and part of the metal pit gantry. He was eventually black flagged by officials.

Boutsen pitted for slicks just bas the rain began to get worse and Patrese stayed out and as the rain intensified he built up a big lead as those who had switched to dry tyres now had to return to the pits to go back to wets. Warwick also stayed on wet tyres and so ran second with Nicola Larini third in the Osella. Senna closed up fast once he was back on wet tyres. Larini's moment of glory did not last long because soon after being passed by Senna the Osella stopped with electrical trouble. Patrese then pitted for new wets and so Warwick was left in the lead although Senna soon caught and passed him. Derek then went out with an engine failure.

In the closing laps as Senna cruised towards the flag, Patrese suffered a problem with his undertray. This allowed Boutsen through into second place and when Senna's engine blew on lap 66 the Belgian found himself en route to his first win. It was also the first victory for Williams-Renault and a 1-2 for the team!

Third place went to de Cesaris with Nelson Piquet (Lotus) fourth and René Arnoux (Ligier) scoring a vital fifth place for Ligier (to help the team escape the threat of having to pre-qualify after the British GP) and Caffi sixth.