[2] In September 2018, Harper signed with Scottish Premiership club Celtic after turning down a college scholarship from UCLA.
[3] After 13 players were ruled out of the first-team squad due to COVID protocols, Harper made his professional debut, appearing as a starter in a 1–1 draw against Hibernian on January 11, 2021.
[4] On March 5, 2021, it was announced that Harper joined Major League Soccer side New York Red Bulls on a transfer from Celtic.
[6] On May 10, 2022, Harper made his first appearance of the season for New York, providing an assist to Zach Ryan in a 3–0 victory over D.C. United as the club advanced to Round of 16 in the 2022 U.S. Open Cup.
[7] A few days later, on May 14, Harper helped New York to a 1–1 draw on the road against Philadelphia Union, assisting Luquinhas on the equalizing goal.