Leissner was a four year starter for the New Hampshire Wildcats. As a freshman, he led the team with 12.3 points and 7.2 rebounds per game and was named the America East Conference Rookie of the Year and second team all-conference.[3] He led the team in scoring and finished as the Wildcats' second-leading rebounder with 15.9 points and 7.3 rebounds per game as a sophomore and was named first team All-America East.[4] Leissner was again named first team all-conference in his junior after averaging 17.1 points and 6.9 rebounds and helped lead New Hampshire to the semifinal of the 2017 America East men's basketball tournament.[5] As a senior, he led the Wildcats for a fourth straight year with 18.7 points per game while also averaging 6.9 rebounds per game. He was named first team All-America East for a third consecutive season and awarded UNH Athlete of the Year.[6] Leissner finished his collegiate career as the Wildcats' all-time leader in points (1,962), free throws attempted (675) and made (521), and minutes played (4,095) while also finishing third in rebounds with 862 in 121 games played.[7]
Professional career
Ehingen Urspring
Leissner signed with Ehingen Urspring of ProA, the German second division, to start his professional career. He finished the ProA season with 16.3 points and 5.4 rebounds over 33 games (32 starts).[8]
On December 1, 2023, Leissner signed with Akita Northern Happinets of the B.League.[16] On April 9, 2024, his contract was terminated.[17] On May 14, he re-signed with Akita Northern Happinets for 2024–25 season.[18]