He has played in the NBA for the New Orleans Hornets, Sacramento Kings, Brooklyn Nets, Boston Celtics, Phoenix Suns, Houston Rockets and Washington Wizards.

As a junior in 2007–08 playing for the Tigers, he averaged 19.6 points (second in the SEC) and shot 81.7 percent from the free-throw line, the second-best percentage in the conference.

As a senior in 2008–09, he averaged 21.1 points, 5.5 rebounds and 2.1 assists per game, subsequently earning the 2009 Southeastern Conference Men's Basketball Player of the Year.

[1] Thornton was selected with the 43rd overall pick by the Miami Heat in the 2009 NBA draft, but his rights were traded to the New Orleans Hornets.

[3] In the lockout-shortened 2011–12 season, Thornton averaged a team-leading 18.7 points, 3.7 rebounds, 1.9 assists, 1.4 steals and 34.9 minutes in starting all 51 games he played in.

[6] On July 10, 2014, Thornton was traded to the Boston Celtics in a three-team deal involving the Nets and the Cleveland Cavaliers.

[17] Two days later, he made his debut with the Wizards in a 114–93 loss to the Utah Jazz, recording two points, one assist and one steal in 20 minutes off the bench.

[19] On February 22, 2017, Thornton was traded, along with Andrew Nicholson and a 2017 first round draft pick, to the Brooklyn Nets in exchange for Bojan Bogdanović and Chris McCullough.

Thornton's fiancée, Lyndsey Duplessis, gave birth to their first child on January 1, 2016, a daughter named Maleia.