On March 4, 2020, Champagnie recorded a freshman season-high 23 points and six rebounds in a 77–55 loss to
Butler. In his next game, three days later, he posted 21 points and 12 rebounds in an 88–86 win over
Marquette. As a freshman, Champagnie averaged 9.9 points and 6.5 rebounds per game. He was a two-time
Big East Freshman of the Week and an All-Freshman Team selection. On November 30, 2020, Champagnie made his sophomore season debut, recording 29 points and 10 rebounds in a 97–93 win over
Boston College. On January 9, 2021, he tallied 33 points and 10 rebounds in a 97–79 loss to seventh-ranked
Creighton, the first 30-point
double-double by a St. John's player since
D'Angelo Harrison in 2015. As a sophomore, Champagnie averaged 19.8 points, 7.4 rebounds, 1.4 steals and one block per game. Following the season, he declared for the
2021 NBA draft while maintaining his college eligibility. However, on July 4, 2021, he announced he was withdrawing from the draft and returning to St. John's for his junior year. On January 5, 2022, he scored a career-high 34 points in a 89–84 win over
DePaul. As a junior, Champagnie averaged 19.2 points, 6.6 rebounds, two steals and 1.1 blocks per game. He was named to the First Team All-Big East as a junior for the second consecutive season. On April 2, 2022, Champagnie declared for the
2022 NBA draft, forgoing his remaining college eligibility. He ultimately went undrafted. ==Professional career==