Mikkola's rally career spanned 31 years, starting with a Volvo PV 544 in 1963, but his most successful period was during the 1970s and 1980s. The 1970s saw Mikkola a frontrunner in many international events, usually in a Ford Escort, and 1979 he made a serious challenge at the World Rally Championship title, ultimately finishing runner-up.
Mikkola was runner-up again in 1980 with Ford, but switched to the new Audi team for 1981, to drive the revolutionary four wheel drive Audi Quattro. The partnership was successful from the outset: Mikkola led the 1981 Monte Carlo Rally, the Audi's first event, until an accident put him out of the event. He won the next WRC event, the Swedish Rally, convincingly but the Quattro had problems with reliability and despite another win on the RAC Rally, Mikkola only managed third in the driver's championship. He won the 1000 Lakes and RAC rallies the following year, but did not improve on third position in the championship, ultimately finishing behind Opel's Walter Röhrl and team-mate Michèle Mouton. 1983 was to be Mikkola's year. Four wins and three second places saw him finally take the World Championship title, with his co-driver Arne Hertz. A second place in the championship followed in 1984, behind his team-mate Stig Blomqvist, but 1985 saw him slip to 22nd in the final standings after the Audi team was overwhelmed by new Group B competition from Peugeot and Lancia.
Mikkola remained with Audi until 1987, winning the Safari Rally in a Group A Audi 200 that year, before switching to Mazda. He remained with the Japanese marque until 1991, then continued to make sporadic appearances on international rallies until retiring in 1993.