Mark Cahill was a high school student taking courses at Greendale Community College for aditional credit. He attends the school along with his close firends Kelly Cortlandt and Scott Waugh. He was a smug and arrogant teenager who, along with his fellow classmates, obnoxiously taunted Jeff and Britta. His first and so far only appearance was in the Season One episode "The Art of Discourse". He is portrayed by actor Jared Kusnitz.
During the study group's first year together at Greendale, a high schooler named Mark Cahill overhears Jeff and Britta's conversation in the cafeteria while eating lunch with his friends Kelly Cortlandt and Scott Waugh. Finding it pathetic that two adults their age was attending the school, he and his friends mock them by calling them "Shmitties", a term used by them to refer to losers. Mark and his cohorts then run across the pair in the library losing even more respect for the duo after seeing that they are hanging out with an even older student. He questions Jeff about how lame his life is and laughs at the fact that he used to be a lawyer. Mark is then further amused when he notices that Britta is carrying a Discman. As they leave, the high schoolers continue to mock the three study group members angering Jeff and Britta.
The next day Mark and his friends are being dropped off at school by his mother Chantelle Cahill unaware they are being observed by Jeff and Britta. Looking to get revenge on them, the duo decides to make fun of the fact that his mom seems to be divorced. It backfires on them as Mark easily deflects their insult by sarcastically mimicking their response. Defeated, the pair retreat only to be followed by the high schoolers who continue to taunt them unabashedly. Later, Mark and his friends run into Jeff and Britta on the quad. They were there with the rest of the study group after a failed attempt by Pierce to properly apologize to Shirley for his recent offenses. Seeing the turmoil in the group only provoked more mocking from the teenagers with Mark derisively mocking that they lost another member after seeing Shirley quitting the group. They go to the parking lot where they are met by Mark's mom who chides them for making her late for her bikini wax.
The following day, Mark, Kelly and Scott are enjoying ice cream in the cafeteria when they see Britta and immediately start to make fun of her. Smiling, she directs his attention to a booth where Jeff is canoodling with Mark's mom Chantelle. Having lost his smug demeanor, Mark goes over to the booth irritated at how low Jeff sank to get back at him. He explains to his mom that he had been "Pwning" Jeff all week and his hitting on her was part of a plan to get revenge. After being exposed and rejected by Chantelle, Jeff and Britta then attempt to take the high road and try to end this rivalry by simply walking away. A taunt from Mark's mom, however, makes Jeff turn back and try one more final attempt to beat the teenagers at their own game.
He walks up to Mark and begins a contest with both sides exchanging the mocking retorts punctuated with the word "A-duh!". Eventually Britta, Kelly and Scott join in as well as the students in the cafeteria along with Chantelle and the rest of the study group watch the bizarre competition unfold. After twelve minutes of this, Jeff and Britta are on the verge of losing when Shirley and Pierce step in and depants Mark and Kelly. Jeff and Britta win by default as the teens fail to reply and Mark is chastised by his mother for being "Pwned". Humiliated by their loss, Mark, Kelly and Scott start a food fight in the cafeteria in order to get revenge. A montage plays out over the food fight, it is revealed through captions that although Mark becomes a doctor, he eventually has his medical license revoked and goes missing afterwards.