At the age of 12, Johnson and AAU basketball teammates Gavin Schoenwald and Darius Garland made the decision to play high school basketball together and chose to enroll in 7th grade at Brentwood Academy in Tennessee. Schoenwald would become a teammate of Johnson's at Vanderbilt, while Garland played hoops at Vanderbilt before becoming the 5th overall pick in the 2019 NBA Draft for the Cleveland Cavaliers.