The car Senna drove to victory was chassis MP4/6/1 — the first McLaren MP4/6 built, which Senna had tested at Estoril in February 1991. Designed by Neil Oatley under the technical direction of Gordon Murray, the MP4/6 was powered by a Honda RA121E 3.5-litre V12 engine developing approximately 720 horsepower at 13,800 rpm and paired with a six-speed manual gearbox. The car was built around a carbon-fibre monocoque. The McLaren MP4/6 would ultimately prove to be the last Formula One car to win a World Championship powered by a naturally aspirated V12 engine with a manual gearbox.
Nigel Mansell, driving for Williams-Renault, led a significant portion of the race after making a strong start and chasing Senna through the early stages. Mansell's stint at the front demonstrated the Williams FW14's pace; he had reduced Senna's lead to under a second before being forced to pit for tyres. By lap 59, Mansell had been fighting his way back toward the lead, setting the fastest lap of the race on lap 60. His challenge ended when his Williams gearbox failed, pitching the car into a spin and retirement on lap 61.
Senna had been conserving resources. By the time Mansell retired, the Brazilian had lost fourth gear in his McLaren — leaving him effectively stranded in sixth gear for the remaining laps. The physical demands of controlling a Formula One car through Interlagos's layout using only a single gear were immense. He was losing time to Riccardo Patrese's Williams with every lap, the gap shrinking as Patrese — who also developed gearbox problems of his own — closed rapidly. Senna held on to win by 2.991 seconds.
The physical toll on Senna was visible to the entire world. After stopping the car at the finish, he could not extract himself unaided and was lifted out by medical personnel. He was driven to the podium rather than walking there, and when he stood on the top step to receive the trophy, he barely possessed the strength to raise it. His post-race comment — "By the finish I had nothing left. God gave me this race" — captured something of the nature of the achievement.
McLaren went on to win both the Drivers' and Constructors' Championships in 1991, with Senna claiming his third title. The Brazilian victory was the opening statement of that campaign — won under circumstances that might have been expected to defeat him, in a car that carried the last of the naturally aspirated V12 engines to claim a world title. The 1991 Brazilian Grand Prix entered the sport's record as one of the defining expressions of endurance and will in modern Formula One.