Thornton hit five 3-pointers and scored a season-high 36 points and the Kings snapped a three-game losing streak with a 120-95 win over the Celtics.
Boston's defense struggled as it allowed a season high in points, including 68 in the second half when the Kings pulled away for a surprisingly easy victory.
The Celtics, who pulled their starters midway through the fourth quarter trailing by 21 points, are 2-2 on their eight-game road trip which continues Saturday night in Denver.
The Celtics had won eight of 10 games, their best stretch of an up-and-down season.
"Whatever you are supposed to have, we didn't have it tonight," Celtics coach Doc Rivers said. "They played terrific and made every shot. Our defense helped. We didn't play with the great spirit that we had in this streak. I don't want this to be a habit."
Prior to the game, Rivers watched on a locker room TV as his son Austin and his Duke teammates were upset by 15th-seeded Lehigh 75-70 at the NCAA tournament.
Although they trailed 52-51 at the half, the Celtics knew they needed more effort in the final two quarters if they wanted to slow down Sacramento.
"We didn't come out with the defensive attitude that we needed to; they got whatever they wanted," Paul Pierce said. "We got a good old-fashioned butt whipping. We have to understand who we are -- we're a defensive group. We got caught up in what they like to do, the running game."
The Kings set a season high for points and ran away from Boston in the third quarter, shooting 70 percent and scoring 41 points in building a 19-point lead. It was their biggest margin of victory in another frustrating season for the Kings, who have the second-worst record in the Western Conference.
The Celtics struggled to contain Thornton all game. He shot 13 of 24, including 5 of 10 3s, and adding four rebounds, four assists and five steals.
"To win games in this league you've got to play night in and night out, and that's what I'm trying to do," said Thornton said, referring to lopsided losses earlier this week to Golden State and Detroit.
Jason Thompson matched his season high with 21 points and had 15 rebounds for the Kings, who are 3-3 on their franchise-record nine-game homestand.
The Kings received a more inspired effort from Cousins, who had been outplayed two days earlier by Detroit's Greg Monroe, who had 32 points and 11 rebounds.
Matched against Kevin Garnett, Cousins and Thompson helped the Kings establish an inside presence and also helped them outrebound Boston 45-28.
"I think we made shots," Thomas said. "We're a young team that feeds off little things like that. If we're making shots, you can say we're going to have a good game."
Ray Allen made five 3-pointers and scored 26 points for Boston. Paul Pierce had 19 points, Brandon Bass had 16 and Garnett added 10 points and nine rebounds. Rajon Rondo had four points and 12 assists.
The Celtics had beaten the Kings in the previous eight games by an average of 17 points.
Ahead by one point at the half, the Kings pulled away in the third quarter when they outscored the Celtics 41-23.
Thornton scored 14 points, Thompson had 10 and Thomas nine for the Kings, who produced their highest-scoring quarter of the season and took a 93-74 lead into the fourth.
"Everything that went wrong with us a couple of nights ago, well it went right with us tonight," Kings coach Keith Smart said. "We moved the ball, we shared it, we ran, we passed, we defended. One thing about this group is they are very, very resilient."
With the Celtics starting to come back from a 17-point deficit midway through the third quarter, Garnett picked up his fourth foul when he pushed Cousins in the paint.
Garnett went to the bench and the Kings went on a run. Thornton scored eight straight points, including consecutive 3s that gave Sacramento a 85-68 lead with just under three minutes left in the quarter.
The Kings opened the third quarter with 13 straight points, including seven by Thomas. The rookie guard hit a 3-pointer and Cousins followed with a tip-in, putting Sacramento ahead 65-51.
Tyreke Evans, the Kings' second-leading scorer, sprained his left ankle in Wednesday's game against Detroit and did not dress for the game.
Veteran John Salmons replaced Evans in the starting lineup. Salmons began the season as a starter, but lost his job after struggling offensively. ... Boston grabbed a season-low 28 rebounds for the third time this season.