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.
Kion Brown led Radford with 13 points and nine rebounds.
The Big 12/SEC Challenge will again feature Kansas and Kentucky, this time at Rupp Arena. That's not the only exciting matchup in next season's midseason conference showdown.
Northwestern has never made the NCAA tournament, but the Wildcats' upward trajectory suggests 2016-17 could finally be the season it happens.
Dillon Brooks, who led Oregon with 16.7 points per game last season, and Tyler Dorsey will opt out of the NBA draft process and return to the Ducks, sources told ESPN.