Toppin redshirted his true freshman season after being ruled academically ineligible to play. As a redshirt freshman, Toppin led the Flyers with 14.4 points per game while averaging 5.6 rebounds per game and was named the Atlantic 10 Conference Rookie of the Week seven times. At the end of the season, Toppin was named the Atlantic 10 Rookie of the Year and to the first team All-Atlantic 10, the first freshman to do so since Lamar Odom in 1999. Following the end of the season he declared for the 2019 NBA draft, but did not hire an agent. After working out for several NBA teams, Toppin opted to withdraw from the draft and return to Dayton. Entering his redshirt sophomore season, Toppin was named to the preseason first team All-Atlantic 10 and to the Karl Malone, Lute Olson, and Naismith Award watchlists. Toppin was also named the 44th-best collegiate basketball player going into the 2019–20 season by CBS Sports and the 24th-best prospect for the 2020 NBA draft by ESPN. He was named the Atlantic 10 Player of the Week for the first week of the season after scoring a career-high 29 points with 12 rebounds in the Flyers' season opening 86–81 win over Indiana State. In late November 2019, Toppin led Dayton to second place at the 2019 Maui Invitational Tournament, averaging 22.3 points, 7.0 rebounds, 2.3 assists and 1.3 blocks over three games. He subsequently repeated as Atlantic 10 Player of the Week and was recognized as National Player of the Week by NBC Sports. On December 30, Toppin scored a career-high 31 points in a 77–59 win over North Florida, including a school-record 10 dunks. He was named to the midseason watchlist for the Wooden Award and was named the Midseason Player of the Year by The Athletic. Toppin sprained his left ankle during a win against UMass on January 11, 2020 but did not miss a game, scoring 24 points in Dayton's next game against VCU in spite of his injury. Toppin scored his 1,000th career point on February 22, 2020 during a 28-point performance in an 80–70 win over Duquesne. At the end of the regular season Toppin was named to the first team All-Atlantic 10 and the Atlantic 10 Conference Men's Basketball Player of the Year after averaging 20 points, 7.5 rebounds and 1.2 blocks per game with a.633 field goal percentage. Toppin was a consensus first team All-American selection, won the Karl Malone Award as the nation's top power forward, and was the consensus National Player of the Year after being named the Associated Press College Basketball Player of the Year, NABC Player of the Year, Naismith College Player of the Year, and awarded the Oscar Robertson Trophy and the John R. Wooden Award. He also garnered national player of the year honors from CBS Sports, The Athletic, NBC Sports and USA Today. After the season Toppin announced that he would be forgoing his final two seasons of eligibility to enter the 2020 NBA draft. Toppin finished his college career with 1,096 points scored and a school-record 190 dunks.