The Lakers had a three-game winning streak halted while falling to the team with the worst record in the Western Conference.
It was the Lakers' first setback under new coach Mike D'Antoni and dropped them back to the breakeven mark.
The Kings led for most of the second half, but the Lakers were within four as the game headed into its final 3 minutes.
Thornton then hit his 3-pointer with 2:50 to play, kicking off a 17-5 Sacramento surge that ended the contest. He hit 8-of-13 shots from the field and scored 23 points off the bench to pace the Kings, who had lost five in a row.
Tyreke Evans added 18 points for Sacramento.
Kobe Bryant scored 38 for the Lakers, hitting 11-of-20 from the field and 11-of-13 from the foul line.