Williamson attended Spartanburg Day School, a small K–12 private school in Spartanburg, South Carolina, where he played basketball for the Griffins. Between eighth and ninth grade, he grew from 5 ft 9 in (1. 75 m) to 6 ft 3 in (1. 91 m). In the summer leading up to his first season, Williamson practiced in the school gym and developed the ability to dunk. At the time, he competed for the South Carolina Hornets AAU team as well, where he was teammates with Ja Morant. As a freshman, Williamson averaged 24. 4 points, 9. 4 rebounds, 2. 8 assists, 3. 3 steals and 3. 0 blocks, earning All-State and All-Region honors. He also led Spartanburg Day to a South Carolina Independent School Association (SCISA) state championship game appearance. In March 2015, Williamson took part in the SCISA North-South All-Star Game in Sumter, South Carolina. By his second year in high school, he stood 6 ft 6 in (1. 98 m). In his sophomore season, Williamson averaged 28. 3 points, 10. 4 rebounds, 3. 9 blocks, and 2. 7 steals per game and was named SCISA Region I-2A Player of the Year. He led the Griffins to their first SCISA Region I-2A title in program history. In June 2016, Williamson participated in the National Basketball Players Association (NBPA) Top 100 camp and was its leading scorer. In August, he won the Under Armour Elite 24 showcase dunk contest in New York City.