He averaged 15 points as a junior, earning Third Team All-Catholic High School Athletic Association (CHSAA) AA honors.

[1] He committed to playing college basketball for St. John's over offers from Pittsburgh, Washington State, Cincinnati and Seton Hall.

[6] On November 30, 2020, Champagnie made his sophomore season debut, recording 29 points and 10 rebounds in a 97–93 win over Boston College.

[7] 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.

[15] On February 10, 2025, Julian played against his twin brother Justin for the first time in an NBA game, when the Spurs met the Washington Wizards.