Thornton scored two first-period goals and completed his first hat trick since March 1, 2000 against the Tampa Bay Lightning 6:47 into the second.
Guerin had successive goals 46 seconds apart in the second, beating goaltender Olaf Kolzig both times to the short side to make it 6-3.
Kolzig was pulled in favor of Craig Billington at 12:07 after giving up six goals on 25 shots.
Thornton got things going at 4:11 of the first. Stationed to the right of the crease, he redirected a point shot by rookie defenseman Nick Boynton past Kolzig.
Thornton made it 3-1 at 12:02 with a power-play goal after assisting on the go-ahead goal by Glenn Murray less than five minutes earlier.
Murray scored his 18th on a snap shot from the top of the slot after Thornton beat Andrei Nikolishin on a faceoff.
Chris Simon benefited from a defensive lapse in front to tie it at 1-1 for Washington just 52 seconds after Thornton opened the scoring.
Thornton, the No. 1 overall pick in the 1997 draft, scored his third goal of the game and 17th of the season off a one-timer from the slot after Sergei Samsonov and Murray gained the offensive zone.
Guerin got on the scoring sheet at 11:21 and again at 12:07 to lift his season total to a team-leading 23 goals. He has eight in the last seven games.
Dainius Zubrus scored a power-play goal with 4:07 left in the first and Simon added his second of the game midway through the second to get the Capitals within 4-3.
Simon escaped down the right side on a breakway with Peter Bondra, exchanged passes and fired up top to Kolzig's left side.
After consecutive goals by Guerin, Boston added another when Rob Zamuner ripped a wrist shot past Billington from the slot for his eighth of the season at 8:21 of the third period.
Washington capped the scoring with 8:24 left as rookie Matt Pettinger cashed in on the power play.
Byron Dafoe made 19 saves for his 19th victory.