Mercer extended the lead to 2-0 when he scored on a backhand off his own rebound from the slot at 7:37.
"We know we're a good team and we want to make sure that we finish the year strong," Mercer said. "We want to make sure that we keep on doing this night in and night out, no matter how many games we have left."
Cole Hutson pulled Washington within 2-1 after collecting a tap-pass from Hendrix Lapierre at the Devils blue line and scoring on a wrist shot from the left hashmarks at 8:15.
Dubois made it 2-2 with a power-play goal off a scramble just outside the blue paint at 9:47.
Dougie Hamilton put the Devils back in front 3-2 on a slap shot from the point at 18:19.
Jack Hughes pushed it to 4-2, scoring on a wrist shot from the left circle off a pass from Bratt from the opposite circle at 9:10 of the second period.
"We've played with each other so much, I know that Jack is usually not too far away from me," Bratt said. "So, I know he's somewhere around there."
Hughes made it 5-2 on a shot from the slot at 8:10 of the third period.
Wilson scored on a snap shot from the high slot to pull the Capitals within 5-3 at 9:33.
"This is a tough game for our group, no doubt," Washington coach Spencer Carbery said. "I feel like we needed to have this. We'll regroup ... I know how we are going to approach it as a staff and I know how our players are going to approach it and we will fight to Game 82, whatever the scenario."
Bratt scored a power-play goal on a one-timer from the right circle at 12:16 to make it 6-3.
Mercer scored an empty-net goal, his 100th goal in the NHL, at 17:53 for the 7-3 final.
"I've always respected my game of playing 200-foot game, trying to capitalize on chances," Mercer said. "Obviously, I've been trying to get that puck in the net and it felt good."
NOTES: Hutson has six points (two goals, four assists) in eight games, matching Timo Blomqvist (six points; five assists, in 10 games) for most points by a Capitals defenseman within his first 10 NHL games. ... Hughes and Bratt became the fourth set of Devils/Colorado Rockies/Kansas City Scouts teammates each with five points in the same game, joining Patrik Elias and Scott Gomez (March 30, 2003), Aaron Broten and Kirk Muller (March 25, 1987) and Kevin Morrison and Rene Robert (Feb. 19, 1980). ... Bratt has 18 points (six goals, 12 assists) in his past 11 games. ... Hischier extended his point streak to six games (two goals, seven assists).