"That is the Strasburg I've known for a long time. That's him," manager Davey Johnson said. "That's what he does. He was very pitch-efficient from the get-go. He went right after guys."
New York dropped its sixth game in a row and 12th in its last 13 games, dropping the club to 11 1/2 games behind Washington in the National League East.
The Nationals, winners of five straight and nine of 12 against the Mets this season, are a season-best 19 games over .500.
"We know we're good. That's what it comes down to," said Michael Morse, one of three Nationals to homer on Wednesday. "Our pitching's really good. Hitters are doing their job, so why not have confidence? Why not have a little swagger?"
Strasburg, unassuming in nature but armed with an overpowering arsenal of pitches, was practically untouchable. He faced three hitters in every inning but the second, fifth and sixth -- when he saw just four. He blew his fastball by plenty of hitters and had others flailing away at his disarming curveball.
"He's tremendous, my gosh almighty," Mets manager Terry Collins said. "The guy last night [Gio Gonzalez] was outstanding. I reviewed the tape this morning from last night, and he was outstanding. ... Electric stuff. Tonight, electric stuff."
Mets rookie Jeremy Hefner, making his fourth career start, took the loss after giving up six hits and three runs (two earned) in six innings. Four of Washington's five runs came on homers, including back-to-back shots by Morse and Danny Espinosa to right field in the second.
It was Morse's seventh of the season and Espinosa's ninth. Espinosa's shot brought the hot-hitting infielder's batting average up to .431 over the course of a 11-game hitting streak after his 2-for-4 performance on Wednesday.
Espinosa also doubled in the fourth and scored on a fielder's choice when Mets catcher Josh Thole mishandled a throw from first baseman Davis on an infield grounder.
Adam LaRoche hit his second two-run homer in as many games with a deep blast to right field in the seventh. It was his team-best 18th homer and gave him 59 RBIs for the season.
Strasburg, coming off a disappointing outing in which he surrendered four earned runs in 5 1/3 innings, is now 5-1 with a 2.63 ERA against the NL East this season.
"It was just, attack the strike zone and don't nibble," Strasburg said. "Just go out there and make them put the ball in play. Obviously, the good pitchers can get through seven in under 100 pitches. So that was definitely a goal. I didn't want to go out there and try and just nibble the whole time and just have 100 pitches through five."
Once Strasburg exited, the Nationals needed four relievers to retire the Mets in the eighth. Henry Rodriguez walked the first two hitters before Sean Burnett and Craig Stammen recorded outs and Drew Storen retired David Wright to end the threat with the score at 5-2.
Tyler Clippard pitched a scoreless ninth for his 18th save of the year.
"This is not an easy place to play," Storen said. "And in the division, we beat each other up, so any time you can get a sweep on the road, it's huge. So now it's important for us to keep it going when we go to Milwaukee."