Mansell joined Williams in 1985, initially partnering Keke Rosberg. He scored his first two Formula One victories that year at the European Grand Prix at Brands Hatch and the South African Grand Prix at Kyalami, announcing himself as a genuine title contender. For 1986, Williams signed Nelson Piquet, the Brazilian two-time world champion who had left Brabham in search of a more competitive machine.
From the outset the relationship was antagonistic. Piquet publicly described Mansell as "an uneducated blockhead" and made disparaging remarks about Mansell's wife Roseanne, later withdrawing them under the threat of legal action. Honda, which supplied the turbocharged V6 engines, reportedly regarded Piquet as the team's de facto lead driver and were said to be paying the majority of his substantial retainer. Frank Williams, who was recovering from a road accident that had left him a tetraplegic following a pre-season test crash, and technical director Patrick Head were committed to running both drivers on equal terms.
The 1986 season was shaped as much by what did not happen as by what did. The Williams FW11 was the class of the field; Piquet won four Grands Prix and Mansell five, and the team clinched the Constructors' Championship comfortably. Mansell also came within 0.014 seconds of Ayrton Senna in the Spanish Grand Prix at Jerez — one of the closest finishes in Formula One history.
The Drivers' Championship went to the final round, the Australian Grand Prix in Adelaide, with Mansell leading the standings ahead of Alain Prost and Piquet. Mansell needed only to finish fourth or higher. Running in third place with nineteen laps remaining, a spectacular rear tyre failure on the main straight ended his race. Piquet, now in a position to win the championship if he took victory, was called into the pits by the team out of concern that his tyres might suffer the same fate. Prost inherited an insurmountable lead and won both the race and the title. Piquet finished second and third in the final standings, one point behind Mansell.
Honda had reportedly lobbied Williams before the season to replace Mansell with Japanese test driver Satoru Nakajima. Williams refused, rightly calculating that two proven race winners gave the team its best chance of Constructors' glory — the championship he valued above all.
The following year brought more of the same — victory at the team level, frustration at the individual level. Mansell won six Grands Prix in 1987, including a celebrated charge at the British Grand Prix at Silverstone where he came from 28 seconds behind in 30 laps to beat Piquet, his car running out of fuel on the slowing down lap in front of the home crowd.
Piquet, however, drove a more calculated championship. He accumulated points methodically, accepting podiums where Mansell might press for outright victory. The outcome was decided not on circuit but in a qualifying accident at Suzuka in Japan at the penultimate race of the season. Mansell struck the armco barrier, suffering a spinal concussion severe enough to rule him out of the final two races. With those points unavailable to him, Piquet became champion for the third time. He was characteristically blunt in assessing his edge over his teammate, stating that his win was "a win of intelligence over stupidity" and attributing his success to consistency where Mansell frequently pushed beyond the limits.
Piquet left Williams at the end of 1987 for Lotus, which had retained Honda engines after the Japanese manufacturer ended its Williams deal. Mansell remained for a difficult 1988 season without turbo power, the team now using naturally aspirated Judd V8 engines. He departed to Ferrari at the end of that year.
Both drivers' legacies are inextricably linked to the Williams years. The rivalry illustrated a central tension in team management — the gain from fielding two aggressive front-runners against the risk they cancel each other out — and produced some of the most dramatic racing of the mid-1980s turbo era.