College career
Thornton became the first William & Mary Tribe signee to be named a Gatorade state player of the year. He chose William & Mary over other schools partly because of its academic reputation and partly because of the attention then-assistant coach Jamion Christian paid him during recruitment. Although Christian ended up taking an assistant coaching job at VCU the spring before Thornton enrolled, it did not affect his decision to attend.
The 2011–12 season, his freshman year, saw the Tribe go 6–26 and finish third from the bottom of the CAA standings. He averaged 11.1 points, scoring 355 on the year, and was named to the CAA All-Rookie Team. He increased his scoring to 18.8 points per game in 2012–13 while he began to establish himself as a player who could create his own shot late in a possession. Thornton was named Second Team All-CAA, becoming just the second W&M player to earn a second team honor in his sophomore year. After scoring 565 points that season, entered his junior year just 80 points shy of 1,000 for his career.
Thornton junior season of 2013–14 was a historic one for both himself and the team. The Tribe recorded 20 wins (20–12 overall), finished third in the CAA, and advanced to their third CAA tournament championship game in seven seasons. The Tribe lost by one point to Delaware, 75 to 74, barely missing out on the program's first-ever NCAA tournament appearance. Thornton's potential game-winning jump shot rattled off the back of the rim as time expired. He was the tournament's high scorer with 59 points in three games and was named to the All-CAA Tournament Team along with teammate Brandon Britt. His 599 total points for the season rank fifth-most in school history and Thornton became just the second W&M player to be named NABC All-District 10 First Team in back to back seasons. He also became the third W&M player to be named First Team All-CAA. Thornton ended the season with 1,519 career points, the most in school history for a player through his first three seasons.
Entering his senior season, Thornton was selected by the league's coaches as the preseason CAA Player of the Year. Lindy's also chose Thornton as the league's most versatile player and its top NBA prospect. He also entered 2014–15 as the only returning All-CAA First Team player with a legitimate shot at breaking the school's scoring mark of 2,052. During the February 18, 2015 game against Northeastern, Thornton surpassed the 2,000-point mark and ended the game with 2,016. The W&M mark of 2,052, set by All-American Chet Giermak, was broken by Thornton exactly one week later in a game against Towson. The school record had stood since 1950 and was the longest-standing Division I men's basketball school scoring record in the country. Thornton guided the Tribe to a 12–6 CAA record as they finished in a four-way tie for the regular season conference championship with James Madison, Northeastern, and UNC Wilmington. On March 5, Thornton was named as the 2015 CAA Player of the Year, becoming the first W&M player to win it in the 32-year history of the award. He also repeated as an All-CAA First Team selection, becoming just the second W&M player to earn the honor twice (joining Adam Hess in 2003 and 2004).
For the second consecutive season, William & Mary earned a berth in the CAA tournament championship game. However, for the first time in school history, the Tribe entered the tournament as the number one seed. They went on to lose the 2015 tournament's final game to Northeastern, 72 to 61, thus ending their chance at a bid into the NCAA tournament. Thornton, however, was named to the CAA All-Tournament Team as well as being the tournament's leading scorer in both 2014 and 2015. By owning the regular season tie-breakers over the other three co-champions, William & Mary earned the automatic National Invitation Tournament (NIT) bid, making the 2015 appearance their third in school history. On March 17, 2015, in the NIT's first round, Thornton scored a game-high 23 points despite the Tribe's 70–67 loss to Tulsa. Thornton finished his collegiate career having amassed 2,178 points, which is the most in William & Mary history and seventh most in CAA history as of the end of the 2014–15 season. He scored 659 points in his senior season, which stands as the second most for a season in school history behind Giermak's 740. He also set the single season three-point field goals made (102) and most career games played (127 – tied) records.
On March 30, the Associated Press named Thornton an honorable mention All-American. He was the first W&M player since 1981 (Mike Strayhorn) to earn the distinction and only the sixth overall to that point.
