Reddish attended The Haverford School in Haverford Township, Delaware County, Pennsylvania as a freshman before transferring to Westtown School in West Chester, Pennsylvania, where he teamed up with Class of 2017 five-star recruit & current NBA player Mohamed Bamba.

As a junior, Reddish averaged 16.2 points per game and led the Moose to a Friend’s School League title. During the 2017 summer, Reddish averaged 23.8 points, 7.6 rebounds and 3.1 assist for his Amateur Athletic Union (AAU) team, Team Final, on the Nike EYBL Circuit. Later that summer, He played for United States men’s national under-19 basketball team during the summer of 2017. He was originally going to play for the under-17 team the previous summer, but did not make the team due to injury. In his senior year, he averaged 22.6 points and 5.6 rebounds per game.

After his senior season, he was named 2018 Mr. Pennsylvania Basketball. Reddish was selected to play in the 2018 McDonald’s All-American Game,  Jordan Brand Classic, and Nike Hoop SummitAll-Star games.

Reddish committed to Duke University to play college basketball, being joined by fellow Top 3 recruits R. J. Barrett and Zion Williamson. On November 6, 2018 In his Duke debut, Reddish scored 22 points in a 118-84 win over  Kentucky at the 2018 Champions Classic. On January 12, 2019, Reddish scored 23 points on 9 of 15 shooting game winner against Florida State. On February 5, 2019, Reddish tallied 24 points and 4 rebounds in a 80-55 win over Boston College. On March 2, Reddish scored 19 points and 7 rebounds in a 87-57 victory against Miami. Reddish appeared in 36 total games for Duke and averaged 13.5 points, 3.7 rebounds and 1.9 assist while shooting 33% from three-point range.

On April 11, 2019, Reddish declared himself eligible for the 2019 NBA draft.