Mouton broke off her contract with Audi in late 1985 to join Peugeot for the coming season. She contested the German Rally Championship and two WRC events in a Peugeot 205 Turbo 16, with which the marque had captured the previous year's world titles. To replace the newly-wed Pons, Mouton recruited Terry Harryman who had been left jobless after Ari Vatanen's accident. Mouton, nicknamed "der schwarze Vulkan" (The Black Volcano) due to her temperament and long black hair, won six of the eight events in the German championship, including the Rallye Deutschland. Although the Hessen Rallye was stopped after the severe accident of Formula One driver Marc Surer, which claimed the life of his co-driver Michel Wyder, Mouton was declared the winner. She secured the German national title on the seventh and penultimate round, the Sachs Baltic, after taking her fifth win of the season. She became the first female driver to win a major championship in rallying.
Mouton states that in her mind she did not try to beat her male rivals, but to be at their level. She noted that in rallying the quickest elapsed time is the most important thing. David Evans of Autosport described her as "motorsport's most successful ever female driver". Rally journalist and historian Graham Robson credits Mouton, along with Pat Moss, as "the driver by whom all other females measure their skills and achievements". Mouton and Moss were of different eras and did not compete directly against each other, although they both appeared at the 1973 Monte Carlo Rally where Mouton co-drove. In 1985, they swapped cars in a private test session with Moss driving the Quattro and Mouton an Austin-Healey 3000. Stirling Moss regarded Mouton as "one of the best", and Niki Lauda described her as a "superwoman".