On July 11, 2015, Harrelson was hired as head coach of the Wayland Baptist men's basketball program. Under his guidance, the team reached the NAIA national tournament four times. He left the program in August 2021. In December 2021, Harrelson was named head coach of
TV Langen of the German Regionalliga. On June 1, 2022, Harrelson was appointed head coach of
SC Rasta Vechta of the German second division,
ProA. In June 2023, he led Vechta to the ProA championship after entering the playoffs as the No. 1 seed. By claiming the championship, the team earned promotion to the German top division,
Basketball Bundesliga. As a newcomer to the league in the 2023–24 season, Harrelson and his Vechta outfit reached the play-off quarter final stages. He moved to fellow Bundesliga side
Ratiopharm Ulm prior to the 2024–25 campaign. He led the Ulm squad to the 2025 Bundesliga finals, where they fell short to
FC Bayern München.
Noa Essengue, one of Harrelson's key players, left to the United States during the finals to attend the
NBA Draft in which he was picked at number 12 overall by the
Chicago Bulls. The same year, a second of Harrelson's players at Ulm,
Ben Saraf, was drafted in the first round. In 2025, Harrelson was a member of
Denver Nuggets’ staff in the
NBA Summer League. ==Personal life==