Lightning McQueen's faithful truck driver, the red tractor-trailer named Mack hauls Lightning from race to race. Good-hearted and unwaveringly loyal, Mack is the one member of Team Rust-Eze that Lightning never manages to alienate.
Mack delivers Lightning McQueen to the Dinoco 400 Piston Cup race, after which he encourages Lightning to speak to his sponsors by backing up his trailer to the Rust-Eze tent. Though Lightning is reluctant to talk to the "rusty old cars," he does his duty and puts in his appearance. Lightning then boards Mack's customized trailer so that he can be taken to the tie-breaker Piston Cup race in California.
Against Mack's better judgment, Lightning convinces him to drive all night, so that he can be the first race car to arrive in California. But Mack can't keep himself awake and starts to drift off, when the four-member street gang known as the Delinquent Road Hazards passes him, and is amused at his attempts to stay awake. DRH member DJ uses his built-in sound system to lull Mack to sleep, and Lightning, who's also asleep in the trailer, accidentally slides out onto the road, without Mack's knowledge. Mack arrives at the race track in California the next day, not aware that his passenger is missing until he opens the trailer with the media's cameras watching.
After Doc Hudson alerts the media that Lightning has been in Radiator Springs for a week while he was missing, Mack joins the reporters in reuniting with Lightning there. Though he's horrified at possibly losing his job for accidentally misplacing Lightning, Lightning is merely happy to see his old friend. Mack takes Lightning to the tie-breaker race and briefly serves as LIghtning's crew chief, until Doc Hudson arrives to take over for him.
Over the end credits of the film, Mack goes to see several movies, including Toy Car Story, A Bug Car's Life, and Monster Cars, Inc. He cheekily muses at how a character in each film is voiced by the same actor, John Ratzenberger. (Ratzenberger also voices Mack.)