Just as the Honda engine began producing regular results, Rosberg decided to leave Williams at the end of 1985 and signed for McLaren, winners of the 1984 and 1985 Drivers' and Constructors' championships. The Williams-Honda team would go on to dominate Grand Prix racing in 1986 and through 1987.
At the time, Rosberg's move to McLaren for the 1986 season had seemed a master stroke as they were the championship team of the previous two seasons, having done so (especially in 1984) in dominating fashion. However, the 1986 McLaren was now somewhat underpowered compared to its rivals, and Rosberg, was soundly beaten by teammate, 1985 World Champion Alain Prost (the McLaren MP4/2C had been designed by John Barnard to suit the smoother style of Niki Lauda and Alain Prost, while Rosberg had never shed the ground effects style of late braking and throwing the car into a corner. It was not until it became known Barnard was leaving for Ferrari that the designer allowed Rosberg to fundamentally change his cars set up to suit his style. Ironically this coincided with Rosberg's only pole position of the season in Germany). On top of that, the fatal crash of Rosberg's close friend Elio de Angelis while testing a Brabham at the Paul Ricard circuit in France in May 1986 deeply affected him and he retired at the end of the season. He would later claim that he retired "too soon".
Rosberg, who had made up his mind in mid-1984 that he would only race for two more years (but did not announce it publicly until Germany 1986), had no regrets about leaving Williams and joining McLaren at a time when the Honda engine was starting to come on strong, while the Porsche built TAG engine (and the 3 season old MP4/2) was starting to show its age. In an interview following his retirement announcement, Rosberg said that he was glad he left Williams when he did, stating that had he stayed with them he might have quit Formula One early in the 1986 season after Frank Williams' pre-season accident (in which he suffered a spinal cord injury which left him a tetraplegic) had left someone in a position of authority within the team who he said was one of the reasons he had decided to leave Williams, adding "We simply could not stand each other". While Rosberg did not name the person, it was generally believed to be Williams head designer and Technical Director Patrick Head, who had taken over the day-to-day running of the team while Frank Williams recovered from his accident.