After attending Sierra Canyon School in Chatsworth, Los Angeles as a freshman, Thornton transferred to Findlay Prep in Henderson, Nevada.

[2] On April 21, 2015, Thornton committed to play college basketball for Duke over offers from Arizona, Kentucky and Louisville.

[3] Coach Mike Krzyzewski convinced him to reclassify to the 2015 class to replace point guard Tyus Jones, who was leaving to play professionally.

[2] As a freshman at Duke, Thornton made 20 starts but saw his playing time fluctuate despite being the only pure point guard on the team.

[7] Thornton averaged 12.7 points, 2.8 rebounds, 3.4 assists and 1.1 steals per game in his final season at Boston College.