Thornton's outstanding four-year career at Florida State University was capped off by a stellar individual senior year that saw him make the AP All-American squad as a third team choice, a unanimous selection as first-team All-ACC, and was runner-up to Boston College's Jared Dudley for the ACC Player of the Year. He led the ACC in scoring and averaged over 7 rebounds per game as well. His season came to a disappointing end as the Seminoles did not qualify for the NCAA tournament for the ninth straight season and were forced to play in the NIT.
On June 28, 2007, Thornton was taken 14th overall in the 2007 NBA Draft by the Los Angeles Clippers. Thornton also has the same agent as Yao Ming. On July 9, Thornton played his first summer league game for the Clippers, scoring 24 points and registering 8 rebounds, 1 assist, and 2 blocks in a 108–102 loss to the Denver Nuggets. In just his third preseason game as a rookie, Thornton led the Clippers to a win over the Golden State Warriors with 24 points on 11–15 shooting, including a perfect 2–2 from behind the 3-point arc while also connecting on all 8 of his first attempts from the floor. With Ruben Patterson in the starting lineup at the start of the season, Thornton would not get the playing time he had received during preseason. Patterson was waived on December 13, 2007 in order to give the rookie more playing time. With a slew of injuries to the Clippers' big men, including Paul Davis with a torn ACL, Tim Thomas with recurring back and ankle injuries, and Elton Brand sidelined with a ruptured achilles tendon, Thornton took advantage of his minutes playing power forward and small forward when teammate Corey Maggette was on the bench. On January 30, 2008, Thornton scored a then season-high 33 points against the Atlanta Hawks, including a significant block in the final seconds in a 95–88 Clipper win. Thornton also recorded his first double-double on February 9, in a loss to the Philadelphia 76ers, in which he had 18 points and 10 rebounds. On March 29, 2008, Thornton equaled a rookie franchise record 39 points on 13 of 23 field goal shooting, 3 of 6 from behind the arch, and 10 of 12 from the line in a 110–97 win over the Memphis Grizzlies. Thornton scored 20 of his 39 in the fourth quarter to seal the win and snap the Clippers' 10-game losing streak. On May 13, 2008, Al Thornton was named to the 2008 NBA All-Rookie First Team. On October 31, 2008, Thornton had a double-double with 30 points and 11 rebounds, a season high.
On March 1, 2011, he reached a buyout agreement with the Wizards and was waived. He signed a contract with the Golden State Warriors on March 3, 2011. On March 14, 2011, Thornton scored 23 points against the Sacramento Kings, his most points as a member of the Warriors.
In March 2014, he rejoined the Guayama Wizards of the Baloncesto Superior Nacional.
Philippines
On January 2, 2015, he signed with the NLEX Road Warriors. In January 2016, Thornton returned to the Philippines to play again for the NLEX Road Warriors for the 2016 Commissioner's Cup.
Japan
On February 28, 2018, Thornton signed with the Shimane Susanoo Magic of the Japanese B.League.
Thornton was tapped to represent the Mighty Sports PH the representative club of the Philippines at the 2016 William Jones Cup. Thornton led the Mighty Sports-Philippines past South Korea on its second day with 24 points in an 86-65 win. Thornton once again led the Mighty Sports by beating India 81-101 with 30 points and on the following day versus Iran 80-73 tallying 24 points and 10 rebounds. Thornton carried the Mighty Sports to a 7-0 win-loss record in the tournament with a double-double 20 points and 15 rebounds by beating Egypt 61-80 and eventually winning the gold medal with an unblemish record of 8-0 by beating Chinese Taipei B 80-104 on the last day of the tournament.
The BIG 3
Thornton had initially gone undrafted in the inaugural BIG 3 draft. Al was later acquired by Allen Iverson's 3's Company" team. Al provided "3's Company" with scoring off the bench and went on to become the league's first "4th man of the year".
NBA career statistics
Regular season
Personal
Thornton is a cousin of fellow NBA player Marcus Thornton. He is the son of Alford and Philomenia Thornton.
