He is a World War II veteran and an old friend of Indiana Jones. By 1957, however, Mac hast lost too much in gambling so he resorts to turn Indy over to Irina Spalko and her men during their quest to find the Crystal Skull of Akator so the Soviet Union can win the Cold War.
He was Indiana Jones' former adventure companion who became a treacherous and greedy agent for Irina Spalko and the Soviet Union. He was previously very loyal to Indy (having even saved him twice during World War II and the Cold War) but became addicted to gambling and would become obsessed with getting rich, which ultimately led to his death.
Mac first appears when the Soviets pull him out of a car he was in with Indiana, outside a hangar in the Nevada desert. He follows his sidekick inside the hangar to help Spalko find an ancient artifact in one of the stored crates. When Indiana grabs a gun in an attempt to escape, Mac betrays him by saying he had only one goal: to get rich. The adventurer manages to escape anyway, and Mac boards Spalko's jeep, who congratulates him.
Mac later traps and captures Indy and Mutt Williams with the Soviets when the duo find the Crystal Skull in the tomb of Fransisco de Orellana. Then they all went to a Russian camp in Brazil to interrogate Indiana. There, Mac tries to convince his old friend to join him by doing as in Berlin but then leaves him in the hands of Spalko. The latter uses the skull to manipulate Jones but Mac is afraid to kill him they stop the operation. While freeing Jones, Mac gets his nose broken and Indiana is taken outside the tent to translate Oxley's incomprehensible words. Although Mutt attempted to make them flee with his mother Marion, Oxley subconsciously alerted Mac and the Soviets who recaptured the group.
After what, the Soviets travel to Akator in the Amazon jungle. As Jones breaks free, he attempts to catch up with Spalko and ends up in Mac's amphibious vehicle which he knocks out. Restrained by Indy, Mac lies to her saying that he is from the CIA and that he is a double agent, allowing him to trust him and sides with him again. He then follows Indy and the others to Akator, leaving behind transponders so that Spalko and his men can easily find them. He flees a group of natives with them and follows them into a room where several crystal skeletons are seated, including the one with the missing skull. That's when he betrays Indy and his friends another time while Spalko rests her skull on the skeleton herself. At this point, the entire room shakes and begins to spin around and Mac begins to flee but returns to retrieve several valuables. Indiana and the others move away from what turned out to be a UFO and Indy yells at Mac to follow him and tries to pull him towards him with his lasso. Mac reassures him that it will be okay and is ultimately killed by being swept away into the vortex that begins to destroy the temple.