Tim Rusthoven added 13 points for William & Mary (6-3).
Ahead 32-31 early in the second half, Thornton made a layup and connected from 3 to push the Tribe’s lead to six, 37-31, with 17:43 left in the period.
Radford (5-5) trailed by single digits most of the second half until Rusthoven’s tip-in and two free throws extended William & Mary’s lead to 10, 57-47 with 3:36 left to play.
The Highlanders made it 58-55 behind R.J. Price’s 3-pointer and Javonte Green’s layup with under a minute to play, but Thornton connected on two free throws with 14 seconds left to seal the victory.
Kion Brown led Radford with 13 points and nine rebounds.