Donohue had a gentle personality which belied his forceful, yet precise style behind the wheel. He won the Can-Am sportscar championship in a Porsche turbo and the Indy 500 in a McLaren, both of which were owned by his longtime friend and mentor Roger Penske. He made his F1 debut in the 1971 Canadian Grand Prix, driving a Penske-owned McLaren, in which he finished third. Duly encouraged, when Penske decided to move Penske Racing into F1 at the end of the 1974 World Championship season, it seemed inconceivable that anyone apart from Mark would be hired to drive the car, even though he was old at 37 to be driving in F1. Penske started in F1 with his own car, but switched to a March 751 in the middle of the '75 season. It was in this car that Donohue crashed heavily following a tire failure in the race morning warm-up for the Austrian Grand Prix. It initially seemed he was only lightly injured, but he died three days later from severe brain injuries.