According to Dale Gribble, Jimmy's condition resulted from staring at the sun for too long, although "He couldn't have been too smart to do something like that in the first place. Kind of a chicken and egg thing". In Ceci N'est Pas Une King of the Hill, it is suggested that Jimmy is inbred, which is another possible explanation for his mental state.
Jimmy has been seen working at a variety of different jobs, most notably managing a concession stand at the race track. During the concession stand gig, he was Bobby's boss, and subjected the boy to verbal abuse, incompetence, or just general weirdness on a daily basis. Despite this, Hank ignored Bobby's complaints about the job, assuming that Bobby was just being a whiner. It was only when Dale told Hank more about Jimmy's past that Hank realized Bobby's complaints were well-founded. The last straw for Hank was when Jimmy nearly killed Bobby by ordering him to cross the racetrack while a race was in progress. Hank, infuriated, ended up running across the track himself, chasing Jimmy down and literally kicking his ass.
In The Substitute Spanish Prisoner, it is suggested that Jimmy is actually an idiot savant with Rain Man -like counting abilities, specifically the ability to count a box of toothpicks. However, this is almost certainly not true, as the character stating it was a con artist whose scam involved tricking people into believing they were geniuses.
- Life In the Fast Lane: Bobby's Saga - First appearance where he was the boss at a concession stand at Arlen Raceway where he was kicked in the ass by Hank for trying to make Bobby give him a soda across the track.
- The Substitute Spanish Prisoner - Jimmy is accepted into a genius society, but it turns out to be a scam, rigged so that anyone can join (much to the consternation of Peggy Hill, who is also accepted into the society).
- Business Is Picking Up - Jimmy is seen bullying Peter Sterling, who owns and operates a very successful business which specializes in picking up animal droppings and cleaning out Port-a-Johns.
- Ceci N'est Pas Une King of the Hill - Jimmy makes art (little people made from beer cans) and is praised by art critics for it. However, when his work sells, he gets angry: "Don't take away my guys!" Jimmy is pacified when the art dealer sponsoring the show promises to give him more cans instead of money.
- When Cotton Comes Marching Home - Jimmy is seen with Cotton, along with Hank watching him and Cotton, getting the money from collecting cans from the manager at Pink & White's recycling center. Cotton disagrees and wants the money, saying that he is broke. Jimmy is seen again later in the episode when Cotton starts to show him "how to count" by throwing cans at him.
- Bill Gathers Moss - Jimmy is seen being interviewed by Hank and the others to see if he would be a good room mate for Bill, but during the interview, he said that he wants the house and a sign outside the front door that says "Jimmy's House".
- Lucky's Wedding Suit - Jimmy is in the lawyers office. He wants to sue somebody because he ate the warning label and choked on it.
- Edu-macating Lucky - Jimmy is seen taking a GED test when Lucky goes to take the test, which whom he is sitting next to, which Lucky says out loud remembering that the value of Pi R squared is 90210, which Jimmy tells him that he just gave away the answer, and covers his test from Lucky.