A three-star recruit, he committed to playing college basketball for Pittsburgh over offers from Cincinnati, Dayton, Seton Hall, Saint Louis and Rutgers.

[9] With Scottie Barnes injured and unable to play,[10] Champagnie was called by the Raptors to make his professional debut on November 1, 2021, scoring his first two points in the NBA on free throws in a win against the New York Knicks.

[18] On August 11, 2023, Champagnie signed with the Miami Heat,[19] but was waived on October 21, prior to the start of the 2023–24 season.

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

[25] Champagnie's identical twin brother, Julian, plays professional basketball for the San Antonio Spurs.