Thornton attended Gonzaga College High School in Washington, D.C., where he averaged 14.7 points, 3.4 rebounds, 4.0 assists and 2.1 steals per game as a senior while maintaining a GPA over 3.3.

Thornton made the all-tournament teams at the Iolani Classic and Alhambra Invitational.

In his senior year he was named to two all-conference teams and the Gonzaga Coaches award in 2008.

Thornton committed to Duke University on September 13, 2008, over Georgetown, North Carolina State, Texas and Virginia.

[1] Thornton joined the Howard Bison men's basketball team as an assistant coach in 2019.