Win No. 51 gave them 108 points on the season, one point above St. Louis' 107 as the Blues were set to face the Dallas Stars on Saturday night.
The Bruins extended their point streak to 15 games, at 14-0-1 with only a shootout loss included in that stretch. With the victory at the Verizon Center, they have won eight straight games on the road, matching the franchise record.
In the past 15 games, Jarome Iginla has led the way in scoring with 13 goals. His most recent pair of goals came in the win over the Capitals. His 30th of the season served as the game-winner.
"It's been fun to win games, and be battling with the top of the league, and to be on some streaks as a group, and to see them go in too," said Iginla.
In that order.
For Iginla and the Bruins, that's the order they operate on - and a testament to their success.
While outside attention rightfully rests on the Bruins' streaks, and records, and the team flip-flopping with St. Louis at the top of the NHL, "playing the right way" is what they constantly focus on. Everything after that, is a byproduct of their play and attitude.
"To us, if you know our group, we don't even talk about that," Bruins Head Coach Claude Julien said, on any discussion of a chance to clinch the division in D.C.
"I knew there was something that would happen if we won tonight, but I found out now that we've clinched and that's fine. Again, we've got an opportunity to do even more than that, so we're just going to keep playing and the main thing is going to be about keeping our game at the level that we want it at."
"Nobody talked about that before the game," Iginla reiterated postgame. "Our focus was to keep going as a group and know that the Capitals would be very hungry in where they are in the standings, and for us, to just match that intensity and that desperation."
Boston expected a strong push from Washington, who entered the day with 80 points in a four-way tie for the two playoff Wild Card spots in the Eastern Conference.
The Bruins didn't necessarily get that from the Caps at the start, and were able to dictate the first period with their smart play in all areas.
They broke through with three straight goals in the second period. After coming out of the penalty box, Carl Soderberg sprung Iginla just before heading off the ice to make a line change with Milan Lucic. Iginla fired it past Braden Holtby to put Boston up 1-0.
Then, in a span of just 41 seconds, Carl Soderberg converted on the power play and Iginla poked through his 30th of the season to give Boston a three-goal lead by the midpoint of the second period.
Jason Chimera would give the Caps life, though, with only 10 seconds to go in the middle frame. They came out in the third with the desperation the Bruins had been anticipating from the drop of the puck.
"There was talk [among us] - you know, we knew how desperate they were," said Iginla. "We know where they are in the standings and how hungry they would be here at home, and to try to match that desperation, and that intensity early. And I thought we did a good job of that."
The Bruins took advantage of a power play given to them in the third period to go up 4-1, with Patrice Bergeron firing in a rebound off a drive from Dougie Hamilton with Soderberg creating havoc in front.
Washington found a way past Chad Johnson on a tough-break goal with 55 seconds left in the game. Johnson went out of his crease to make a pokecheck, and the Capitals were able to get the loose rebound into the back of the net.
"If you don't look at the score, they gave us a really good game, and it was a lot closer than the score indicates," said Julien, of his squad's 4-2 final.
"I thought all the way around, we played a really solid road game," said Iginla. "Goaltending was great again. With Johnny coming in there, he had some huge saves early, our PK was really good - they have a great power play and it was a big boost for us to get some of those kills - and then our power play was good, too."
Johnson made 31 saves in the win and has won his past six starts with a 1.33 goals-against average, .951 save percentage and one shutout during that stretch.
The Bruins will now prepare to face another desperate opponent in the Flyers on Sunday in Philadelphia to finish off the road back-to-back.
"That's what keeps us on our toes," said Julien. "We're enjoying that because even though we've clinched a playoff spot, there's more for us there if we want, and at the same time, it's keeping us on our toes, keeping us sharp, and we don't want to be taking these last few weeks here and just coasting through, because those bad habits can hurt you down the road."
As the Bruins keep preaching every day, they welcome those challenges.
"Sometimes you've got to put yourself in the other team's shoes, you've got to put that desperation into your own game, individually and as a team," said Lucic, of how to counter teams like Washington and Philadelphia down the stretch.
"You always want to strive to get better. If there's any time to want to get your game to its peak and to an A game, it's right now."
Even with the Atlantic Division title checked off on the Bruins' list, there still won't be any change in their approach.
"Yeah, we sort of just keep checking them off," said Johnson. "For us though, we just worry about the games at hand, and the wins, and let everything else take care of itself."