By Lord Hesketh could no longer afford to run the team and sold out. Postlethwaite went with his cars to the newly founded Wolf-Williams team, headed by Walter Wolf and Frank Williams, but the results were poor and the owners soon went their separate ways. Postlethwaite remained with Wolf, designing the team's challenger, the WR1.
Success was immediate with Jody Scheckter taking victory at the season's opening race. Two more wins and a number of podium results followed and Scheckter eventually finished second in the drivers' championship.
Although Postlethwaite remained with the team until they were never to repeat their success. When Walter Wolf closed the team down at the end of 1979 he transferred, along with the Wolf cars and driver Keke Rosberg to the Fittipaldi Automotive team. He produced a new design, the F8, for the latter half of 1980 but left to join Ferrari in early 1981. At the time the Italian team were considered amongst the best engine builders in the sport, but amongst the worst chassis designers. Postlethwaite was selected personally by Enzo Ferrari to rectify this problem and by the following year everything was in place for success.
The 126C2 Ferrari took the constructors' title despite several serious setbacks, including the practice crash at Zolder which claimed the life of Gilles Villeneuve. Despite the loss of their inspirational Canadian driver, Postlethwaite's updated design, the 126C2B, took the constructors' title again in .
Postlethwaite remained with Ferrari until . After 1983 his cars took several more wins, but were unable to compete with McLaren and Williams for title victory. He was eventually replaced by John Barnard and moved to Tyrrell, where he worked for four years. During his tenure as technical director Tyrrell's results improved noticeably, culminating in the season opener in Phoenix, where Jean Alesi was able to challenge Ayrton Senna's McLaren for victory and finished second in a Tyrrell 018. Alesi repeated the feat in the Postlethwaite's novel 019 - the first of the 'high nose' Formula One cars - at Monaco. At the car's launch Postlethwaite proved the structural integrity of its unusual front 'gull wing' by standing on it. While at Tyrrell Postlethwaite employed Mike Gascoyne, who became his assistant and protégé.
In 1991, Postlethwaite was signed as technical director of the Sauber team who planned to enter Formula One in . Taking Gascoyne with him, Postlethwaite relocated to Switzerland and designed the team's first car. Despite leaving Sauber before the start of 1993, the designer's car went on to considerable success in the hands of JJ Lehto and Karl Wendlinger regularly scoring points.
Postlethwaite moved back to Tyrrell in 1994 where he remained until when the team was sold to become British American Racing. Although by the late 1980s and 1990s Tyrrell was a small, and largely uncompetitive team, the designer remained well respected within the sport and was hired as technical director of the abortive in-house Honda F1 project in 1999. Although Honda had not committed to race in Formula One the project produced an evaluation car, designed by Postlethwaite and built by Dallara, and it was during testing of this car at Barcelona in Spain that he suffered a fatal heart attack. The project was subsequently discontinued, although Honda began supplying engines again from the 2000 season onwards, eventually taking over the BAR team for 2006.