Doc is Radiator Springs' resident judge and doctor. He was once one of the most famous racecars in history; he won three Piston Cups, and still holds the record for most wins in a single season. However, it all changed for Doc when he was involved in a terrible crash during the final lap of the 1954 Piston Cup championship race, which saw him put out for the season.Upon his return, he was received with a complete absence of fanfare and told that he was a has-been who had been passed up for the next rookie in line. He kept a newspaper article on the career-ending crash as a reminder never to return to the life that nearly killed him.
Jaded by the racing scene, he left that world, apparently taking out time to study medicine. The famous #51 disappeared into obscurity, leaving many wondering where he had gone. He instead opted for a simple navy blue paint job and the life of a physician in the tiny town of Radiator Springs, the "shining Gemstone" of the Mother Road - Route 66. He runs Doc's Clinic as a "doctor of internal combustion". As times changed and the town got bypassed in favor of Interstate 40, Doc stayed on, even when the population had dwindled to a meager dozen or so residents. He is respected, well-loved, and serves not only as the town's physician, but as its judge as well. However, nobody in the town had any idea of his past as a racer; he was just an ordinary Hudson Hornet to them.
Upon meeting Lightning McQueen, Doc saw far too much of his past in the rookie. He was less than happy when Lightning discovers his past. After Lightning finished fixing the road he damaged and decided to stay for a while, Doc was unable to bear having him around any longer and called the news and press to immediately take him away to the Piston Cup, declaring that it is best for everyone. But seeing how disheartened everyone was by his unplanned departure, Doc realized that Lightning had become more important to them than he thought. He eventually admits the truth to everyone about his Racecar days and he took back his old #51 colors to become Lightning's pit crew chief, bringing nearly the entire town to the Piston Cup to support Lightning as his pit crew and in an ironic twist of fate, finally received that long awaited fanfare for his return.
By the end of the film, Doc opts to keep his racing colors, and becomes a trainer as well as a friend to Lightning. Just like Lightning, Doc learned some lessons: friendship, promises, how greed affects others, and that secrets can't be kept forever.
Doc died prior to the events in Cars 2, but posters of him are still shown in his museum. In memory of him, the Piston Cup was renamed the Hudson Hornet Memorial Piston Cup,and gets a new logo with an image of the Hudson Hornet and the words "Hudson Hornet" above it. It appears prominently on Lightning McQueen's hood . Doc’s office was converted into the Hudson Hornet Racing Museum.
When Lightning was showing Mater his new Piston Cup, they mentioned him a few times. Also, during the race in Tokyo, on the dirt section, Mater told Lightning to do what Doc has taught him. Which he did and took the lead.