The Celtics had led 2-0 and 3-2 in the best-of-seven contest but were twice pegged before a 115-105 win in game seven sent them through to a meeting with the Cleveland Cavaliers.
The Celtics were led by their star guard Isaiah Thomas with 29 points and a game-high 12 assists.
Kelly Olynyk was the unsung hero for the Celtics, making 10 of 14 shots and scoring 26 points off the bench.
Beal led the Wizards, who have not reached the conference final since 1979, with 38 points, Otto Porter scored 20 points and grabbed 10 rebounds and John Wall had 18 points, 11 assists and seven rebounds.
The Celtics came to Washington a few days ago wearing all black, dressed in their "funeral" attire as they attempted to close out the series and end the Washington Wizards' season. Overall, the Celtics' bench outscored the Wizards' 45-5, which showed the difference in depth between the two teams.
Boston, the top seed in the Eastern Conference, will now have home-court advantage in the Eastern Conference finals against LeBron James and the mighty Cleveland Cavaliers. His buckets made Celtics fans giddy, as they celebrated a level of athleticism they rarely see out of their long-locked 7-footer.
From there, the Celtics could taste the Eastern Conference finals and weren't about to let the opportunity slip away. The Celtics will try thick, wide-bodied Jae Crowder, and the Cavs will run James through pick-and-rolls when they do.
John Wall put on his sunglasses and walked slowly toward the team bus after the loss with a bitter taste in his mouth, albeit a different flavor despite losing for the third time in the past four years at the same spot on the same date: the Eastern Conference semifinals on May 15.
"They didn't give us a chance [against the Wizards]". "Golden State is more of an outside shooting team and they're very good at it". Maligned for their play throughout the month of March and beginning of April, the Cavaliers converted 43 of their 92 attempts, including 11 three-point attempts and 17 of their 18 free throws in the lopsided win.
Jaylen Brown only played 19 minutes in Game 7 against the Wizards, but they were certainly impactful minutes.
Olynyk was the victim of a Kelly Oubre Jr. shove in Game 3 that got the Wizards guard suspended for Game 4.
Olynyk had entered the game averaging only 8.3 points per game in the playoffs but delivered a career-high playoff performance when it mattered. "I think they're playing better this year than they did last year".
As the series went on, Beal's play seemed to get better and better.
"I knew it wouldn't be easy at all", All-Star guard Thomas said. If it's the Celtics, it will be on the back of a Thomas masterclass. The Wizards are 6-4 in Game 7s, 2-4 on the road.
Olynyk's highlight basket of the night came during the first quarter, when he faked a dribble-hand-off to Thomas on the right wing and turned the corner for a drive to the rack that eventually turned into a three-point play.