The Game 7 result eliminated the defending Stanley Cup champion Bruins, who were trying to become the first team to win back-to-back crowns since Detroit in 1997-98. Boston won three Game 7's in last year's playoffs, including the championship clinching victory over Vancouver.
Thomas was the hero of Boston's march to the Stanley Cup in 2011, but this year he was outplayed by Washington's Braden Holtby.
Injuries to both of Washington's front-line goaltenders forced Holtby into action after he had played in just seven regular-season contests. He made 31 saves against the Bruins Wednesday, leaving him with a 2.00 goals-against average and a .940 save percentage in the opening round of the playoffs.
The winning goal came after Mike Knuble knocked down a pass in his own zone and brought the puck into the Boston end down the left wing. He charged the net only to have Thomas stop his backhand shot.
Thomas could not prevent a rebound, however, and Ward was there to bang it into the net.
Washington's first goal came on a tip-in from Matt Hendricks in the opening period. Tyler Seguin tied it with 5:33 to go in the second.
This was the first of three seventh games to be contested in the opening round of the playoffs. Ottawa will be at the New York Rangers and New Jersey will be at Florida Thursday in the other concluding games.
Washington's opponent in the conference semifinals will depend on Thursday's results.