Kankkunen's path to the championship began with Toyota Team Europe, where his performances from 1983 to 1985 earned him a move to the defending champions Peugeot. Replacing Ari Vatanen, who was still recovering from a near-fatal accident, Kankkunen drove the Peugeot 205 Turbo 16 E2 to victories in Sweden, the Acropolis Rally, and New Zealand. The title came amid controversy: Peugeot were excluded from the Rallye Sanremo due to a ruling that their cars' underbody fins constituted illegal side skirts, briefly handing the championship lead to Lancia's Markku Alen. Peugeot's appeal succeeded and the FIA annulled the Sanremo results, restoring Kankkunen's title. At 27 years old, he became the youngest champion in the history of the series.
When Peugeot withdrew following the FIA's ban on Group B cars — triggered by Henri Toivonen's fatal accident at the 1986 Tour de Corse — Kankkunen moved to Lancia Martini to drive the Delta HF 4WD. He became the first driver to successfully defend a WRC drivers' title. The season was not without friction: Lancia team boss Cesare Fiorio controversially instructed Kankkunen to finish second to teammate Miki Biasion on his debut for the team at Monte Carlo. Despite the team order politics, Kankkunen won the Olympus Rally and clinched the title at the season-ending RAC Rally.
After two seasons at Toyota from 1988 to 1989, Kankkunen rejoined Lancia for 1990, finishing third as Carlos Sainz took the title. The 1991 season proved his strongest. Kankkunen won the Safari Rally, the Acropolis Rally, the 1000 Lakes Rally in his home country for the first time, and the Rally Australia for the third consecutive year. Going into the RAC Rally, Toyota's Sainz led him by a single point. Kankkunen won the RAC ahead of Kenneth Eriksson and Sainz, claiming a record third title. His 150 points across the 1991 season remained the record for most points in a single WRC season. The feat made him the first driver to win three championships since the series began in 1973.
After Lancia withdrew from the WRC at the end of 1992, Kankkunen rejoined Toyota to drive the Celica GT-Four ST185. Mid-season, regular co-driver Juha Piironen suffered a brain haemorrhage and Kankkunen completed the year with substitute co-drivers. Despite this disruption, he won five of his ten events: the Safari Rally, Rally Argentina, Rally Australia, 1000 Lakes Rally, and the RAC Rally. The RAC victory was his twentieth career WRC win, breaking compatriot Markku Alen's record for the most wins in the championship's history. His fourth title was also the first WRC drivers' championship won by a driver competing for a Japanese manufacturer's factory team.
Kankkunen's four titles were matched by Sebastien Loeb, who first equalled the record in the 2000s and then surpassed it. Ogier later joined Loeb as a driver with more titles. But Kankkunen's cross-manufacturer achievement — winning with Peugeot, Lancia twice, and Toyota — remained unique for decades. He also won 23 world rallies in total during his career, a record at the time. Outside rallying, Kankkunen won the Dakar Rally in 1988 and the Race of Champions in 1988 and 1991. After retiring from competition at the end of 2002, he has remained connected to the sport: since the 2025 WRC season, he has served as team manager of Toyota Gazoo Racing WRT.