McDermott was born Nov. 3, 1996 and is the son of Mike and Kim McDermott. He has one brother, Chayce, who was a baseball pitcher at Ball State. In high school, Sean played for Pendleton Heights. While in high school, McDermott suffered from a life-threatening staph infection. Following his senior campaign -- during which he posted 16.0 points, 6.6 rebounds, and 2.0 assists per game -- he was named an Indiana All-Star. McDermott played four collegiate seasons at Butler after redshirting his freshman campaign. He started all of his appearances during his junior and senior campaigns and was known for his accurate outside shooting. As a senior, McDermott averaged 11.7 points, 6.3 rebounds and 1.0 assists. Following that season, he declared for the 2020 NBA Draft. McDermott redshirted his 2015-16 freshman season at Butler. He made his debut for the Bulldogs in 2016-17, averaging 2.3 points and 1.4 rebounds in 10.7 minutes per game. McDermott upped his playing time as a sophomore, averaging 7.5 points, 3.9 rebounds and 1.0 assists in 23.8 minutes per contest. McDermott became a full-time starter as a junior, averaging 9.6 points and 3.9 rebounds in 27.9 minutes. Finally, as a senior in 2019-20, he averaged 11.7 points, 6.3 rebounds and 1.0 assists in 32.9 minutes per contest. As a senior, he led the NCAA in Offensive Rating with a score of 132.6. Throughout his four year college campaign, McDermott hit 40.3 percent of his 496 three-point attempts.