During Mark's time developing the turbocharged Porsche 917/10 TC at Road Atlanta, he would occasionally come to Peachstate meetings, much to the thrill of the membership, so Peachstate made him an honorary member.
Between 1972 and 1973, Penske Racing (along with Donohue as the primary test and development driver) was commissioned by Porsche to assist with development of the 917/10. Donohue extensively tested the 917/10, offering up his substantial engineering knowledge to the Porsche engineers in order to design the best possible race car to compete in the Can-Am series. Donohue's desire to succeed almost led to his undoing. During testing of the 917/10 at Road Atlanta, Donohue had recommended larger brake ducts to the Porsche engineers, in order to provide more efficient cooling, and thus less fade and degradation as a race wears on. The Porsche engineers obliged, but in doing so, caused the new brake ducts to interfere with the bodywork closure pins, which attach the bodywork to the car. Coming out of turn seven, the rear bodywork flew off the car at approximately 150 mph (240 km/h), causing the car to become extremely unstable. The car lifted off the ground and tumbled multiple times down the track. The front of the car was completely torn away, leaving Donohue, still strapped to his safety seat, with his legs dangling outside the car. Amazingly, Donohue only suffered a broken leg. George Follmer, Donohue's old Trans-Am teammate, resumed testing the 917/10 while Donohue was on the mend. In classic Donohue style, Donohue said of Follmer testing his car: "It just doesn't feel right. Seeing another man driving your car, a car you know so well. I imagine it must feel like watching another man in bed with your wife."
Porsche, Penske and Donohue quickly started the development of the 917-30, complete with a reworked aerodynamic "Paris" body and a 5.4-liter turbocharged flat-12 engine whose output could be adjusted between approximately 1100 and 1600 bhp by turning a boost knob located in the cockpit. During the development of this motor, the German Porsche engineers often asked Donohue if the motor finally had enough power. His tongue-in-cheek answer was "it will never have enough power until I can spin the wheels at the end of the straightaway in high gear."
The Porsche 917/30 that Mark drove is erroneously referred to as "The Can-Am Killer" as it dominated the competition, winning every race but one of the 1973 Can-Am championship. However, the SCCA imposed fuel limitations for all Can-Am races due to the existing Arab Oil Embargo. Because of this, Porsche and McLaren withdrew from the series. It is generally considered one of the most powerful and most dominant racing machines ever created.
Donohue, Penske and Porsche decided to set their goals very high with the 917-30. After making various aerodynamic and suspension modifications to the car, Donohue set the then world closed-course record driving the CAM2 Motor Oil sponsored Porsche 917/30 at the Talladega Superspeedway in Talladega, Alabama on August 9, 1975. His average speed around the 2.66-mile (4.28 km) high-banked oval was 221.120 mph (355.858 km/h). Donohue held the world record for eleven years, until it was broken by Rick Mears at Michigan International Speedway.
The following quote has been attributed to Donohue: "If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough horsepower."
Midway through the 1975 Formula 1 season, Penske started testing the March 751 racecar. Mark Donohue had recently arrived in Austria for the Austrian Grand Prix at the ?sterreichring race track following the successful closed-course speed record attempt at Talladega Superspeedway in Alabama just a few days earlier. During a practice session for the Gran Prix race, Donohue lost control of his March 751 after a tire failed, sending him careening into the catch fencing. Donohue's head was said to have struck either a catch fencing post, or the bottom of the wood frame for an advertising billboard located trackside. A track marshal was killed by debris from the accident, but Donohue did not appear to be injured significantly. However, a resulting headache worsened and after going to the hospital of Graz the next day, Donohue lapsed into a coma from a brain hemorrhage and died.
Mark's racing legacy has been carried on by his son, David, who himself is an accomplished Porsche racer, winning the 2009 24 Hours of Daytona driving a Brumos Porsche-entered Riley-Porsche, beating Juan Pablo Montoya by the closest margin in the race's history.