Mitch "Assface" "Penis Breath" joins the Cobra Kai Dojo and quickly adopts its philosophy, becoming a bully in the process. However, after being kicked out of the dojo for losing to a newly recruited Kyler, which results in him being rejected by his former friends, he joins Eagle Fang Karate, and later attends the first meeting of the combined Eagle Fang and Miyagi-Do Karate dojos, redeeming himself in the process.
He trains under Sensei Johnny Lawrence and the latter's Sensei John Kreese. He gets exposed to Kreese's ruthless teachings. While Miyagi-Do Karate student Demetri is at the comic book store, Mitch along with other Cobra Kai students which includes Chris and Hawk surround him. Demetri tries to run away but Mitch along with the others surround him. However, Miyagi-Do students Robby Keene and Samantha LaRusso beat all of them up.
After Chris quits Cobra Kai along with some other students, Mitch becomes upset. Chris's decision to quit ends their friendship
Mitch fights Chris during the school brawl. He is eventually is able to kick Chris to the ground, but Chris wins the fight by clubbing him on the head with a school book.
When Johnny goes back to Cobra Kai and sees Kreese teaching a handful of his students, Mitch is one of the students present, indicating that Mitch has turned against Johnny.
Mitch remains in Cobra Kai under Kreese's control. He even has a haircut this season. He remains a close friend to Hawk throughout the first half of the season. After Hawk finds out about Miyagi-Do hosting a car-wash to raise money for supporting Miguel's surgery, Mitch helps Hawk to confront Nathaniel and take the money they had gathered. Mitch comments that the money will be donated in the name of Cobra Kai.
Later on, Mitch can be seen with Hawk, Mikey and Doug, causing havoc in the local arcades. The group ends up breaking into a nearby laser tag arena to drink beer, where they are attacked by Miyagi-do students. Mitch ends up fighting Chris and initially finds himself on the losing end. However, as Tory arrives with two other Cobra Kai students to help, Mitch manages to get help from Mikey and Doug. The three eventually defeat Chris together.
When Kreese brings new recruits to Cobra Kai, Mitch is ordered to spar with Kyler. Kyler quickly taps Mitch out with a choke. Kreese responds to the loss by kicking Mitch out of Cobra Kai. Hawk protests Kreese's decision, stating that Mitch has been loyal to the dojo, but Kreese states that Mitch is not Cobra Kai material.
After being kicked out of Cobra Kai, Mitch finds himself rejected and mocked by Hawk and his former classmates, but shortly afterward he joins Johnny's newly-founded Eagle Fang Karate along with Miguel, Bert, and a few others. Mitch is present when Kreese and the rest of Cobra Kai confront Johnny during their training, offering Johnny a last chance to re-join the dojo.
Mitch gets invited to the LaRusso house along with the rest of Eagle Fang to discuss working together with Miyagi-do against Cobra Kai. Mitch protests the offer at first, but Demetri manages to convince him that it would be for the greater good. The meeting is attacked by the Cobra Kai students, and Mitch helps defend the house during the ensuing brawl. He spends most of the time fighting Mikey, and overall has the upper hand. However, he is later seen being beat up by another Cobra Kai, and then fighting Doug. In the end, Mitch and his side prevails, partly due to Hawk defecting and joining his side.
Following the house fight Mitch is present as Miyagi-do and Eagle Fang join forces. Mitch and Chris can be seen shaking hands and putting an end to their rivalry, as Mitch takes his place in formation to start training under Daniel's and Johnny's guidance.
Shortly after joining Cobra Kai, Mitch is corrupted by Kreese's teachings, which makes him vicious, aggressive and a bully. When his friend Chris quits Cobra Kai, their friendship ends. During the school brawl, Mitch has no problem attacking Chris. By the end of the season, Mitch along with several other students have turned against Johnny and become Kreese's students.
He causes trouble alongside his friends, such as taking the money for Miguel's surgery from Nathaniel and beating him up, (though Mitch says he'll make sure Miguel gets the money and surgery), as well as creating trouble and fighting with the Miyagi Do's in the arcade. However, after being kicked out by Kreese for losing to an untrained Kyler, leading him to be shunned and mocked by his former classmates, he joins Johnny's Eagle Fang Karate Dojo. Upon being requested by Sam for Miyagi Do and Eagle Fang to work together, Mitch protests and proceeds to leave, but ultimately agrees to the truce after Demetri gives a speech about how they need to work together to beat Cobra Kai. The following morning, both Mitch and Chris have rekindled their friendship, and he begins training at Miyagi Do with Chris, and other Miyagi Do and Eagle Fang students.
Mitch's skills are rather limited. After joining Cobra Kai he gains some basic striking ability and has enough power to knock down the larger Chris. He is aggressive but reckless, and his defensive skills are fairly weak. He has no ground game at all, tapping out immediately when Kyler locks him in a choke hold.
During his fight with Chris in the school fight at the end of season 2, the latter has the upper hand, until Mitch eventually kicks him to the ground. However, Mitch then loses when Chris knocks him out with a book. During the arcade fight in season 3, Mitch and Chris fight, with the latter's more solid defense and technique allowing him to gain the upper hand, until Mitch eventually gets assistance from Doug, which results in both Mitch and Doug defeating Chris. During his fight with Mikey at Sam's house at the end of season 3, Mitch overall has the upper hand.
- Mitch was one of the Cobra Kai students that trashed the Miyagi-Do dojo.
- Mitch appears to be a professional wrestling fan, as seen in his debut, in which he wears a WrestleMania shirt.
- Mitch prefers the nickname “Ass Face” over “Penis Breath,” prompting Chris to ask him why he would put up with insulting nicknames at all.