As a freshman at Kansas, McCormack averaged 3.9 points and 3.1 rebounds per game.[8] On December 14, 2019, he recorded a sophomore season-high 28 points and seven rebounds in a 98–57 win over UM Kansas City.[9] As a sophomore, McCormack averaged 6.9 points and 4.1 rebounds per game.[10] He became a regular starter in his junior season with the departure of Udoka Azubuike.[11] On January 12, 2021, McCormack posted 24 points, 12 rebounds and three blocks in a 75–70 loss to Oklahoma State.[12] In the NCAA Tournament, he scored 22 points and posted nine rebounds in a first-round victory over Eastern Washington despite playing through pain. As a junior, McCormack averaged 13.4 points and 6.1 rebounds per game, shooting 51.5 percent from the floor. After the season, he underwent surgery to repair a broken bone in his right foot.[13]
In his senior year, on January 4, 2022, McCormack scored 17 points and achieved a career-high 15 rebounds in KU's defeat of Oklahoma State.[14] On January 22, 2022, he got 15 rebounds against Kansas State, tying his personal best in rebounds. He averaged 10.6 points and 6.98 rebounds per game in his senior year. McCormack was named to the Third Team All-Big 12 as a senior.[15] On April 2, 2022, in the 2022 national semifinals against Villanova in New Orleans, McCormack scored 25 points and grabbed 9 rebounds (along with one assist and one steal), leading KU to an 81-65 victory, sending Kansas into the national championship game against North Carolina.[16] In the national championship game against North Carolina on April 4, 2022, McCormack scored 15 points and grabbed 10 rebounds (along with one block and one steal), leading Kansas in the largest comeback in NCAA Tournament history.[17] McCormack was named to the all-tournament team for his play in Kansas' six tourney games, where he averaged 13.1 points a game.[18]