Ovechkin, who was held without a goal for the second straight game, remains two from becoming the eighth player to score 700 NHL goals.
"I think we came back and tried to take his time and space," said New York coach Barry Trotz, who coached Ovechkin with the Capitals from 2014-18. "Ovi's so good. He doesn't need much of a window. I've seen it many times. He's going to get 700. It wouldn't have bothered me if he got 700 as long as we won the hockey game. I know he's going to get it, and I'm a big believer he's going to break [Wayne Gretzky's NHL record of 894 goals]. I wish him luck because we don't play them anymore for the rest of the year."
Anthony Beauvillier scored twice, and Josh Bailey and Brock Nelson each had three assists for the Islanders (32-16-6), who are 4-1-2 in their past seven games. Thomas Greiss made 25 saves.
[WATCH: All Islanders vs. Capitals highlights]
"I thought the top guys delivered tonight," Trotz said. "They were a little quiet for us, and they delivered tonight, and they're going to have to deliver down the stretch."
John Carlson had a goal and an assist for Washington (36-15-5), which has lost three of its past four games.
"We found a way to win a lot of games maybe when we shouldn't have earlier in the year, and now we're finding a way to lose them," Capitals coach Todd Reirden said. "So now we need to figure out, as a group, together, how to solve this."
Video: NYI@WSH: Beauvillier buries one-timer on the rush
Ilya Samsonov allowed five goals on 20 shots before being pulled for the first time in his NHL career and replaced in the second period by Braden Holtby, who made 12 saves.
"It's always tough when it's your first time going through something like that," Holtby said. "The thing is, he had no chance -- it was just stacked against him. It's a big thing. He has a really good head on his shoulders, and I'm sure he knows that."
The Islanders took a 1-0 lead at 3:30 of the first period when Beauvillier scored on a 3-on-2 rush with a one-timer from the right circle off a pass from Bailey.
Beauvillier made it 2-0 at 6:16 when he scored off the rebound of a Nelson shot during a scramble in front.
"You kind of find your legs when you get points early," Beauvillier said, "and I thought we as a group just did a good job sticking with it and following the game plan, just doing the right things out there."
Beauvillier has five goals and three assists during a six-game point streak.
"I think he's just playing free and having fun and he's getting good looks," Nelson said. "He's got all the tools you need to play and be a great player in this league, and he's showing that."
Carlson scored with a shot from the point that went in off defenseman Johnny Boychuk at 12:55 to cut New York's lead to 2-1.
Defenseman Devon Toews made it 3-1 when he got behind the Washington defense to complete a give-and-go with Bailey at 18:06.
Leo Komorov pushed the lead to 4-1 with a wrist shot from the right face-off circle 45 seconds into the second period, and Jordan Eberle tapped in a pass from Anders Lee on the rush to make it 5-1 at 10:36, chasing Samsonov.
The Capitals, who came back from down 4-1 in the third period of a 6-4 win at the Islanders on Jan. 18, made it close, but their rally fell short.
Lars Eller put in the rebound of Carl Hagelin's wraparound at 11:25 to make it 5-2, and T.J. Oshie got Washington within 5-3 with a one-timer from the left circle on the power play at 16:59 of the second.
"A couple games lately we found ways to get back, and tonight I think we did a great job at trying to get back, but you know, ran out of time or the deficit was a little too big," Oshie said. "So we are struggling obviously a little bit, but you know, we can learn from a lot of these mistakes and grow and try to get better."
Capitals forward Evgeny Kuznetsov left the game at 9:14 of the second period with an upper-body injury after he was hit into the boards in front of the Islanders bench by Komarov. He's day to day.
Video: NYI@WSH: Eberle finishes off the feed from Lee
They said it
"[They] get it to 5-3 in the second, and I'm sure people were thinking back to that last game (when the Capitals scored five straight goals to rally for a 6-4 win Jan. 18), but we did a good job coming back in the third. We won some battles along the boards. It doesn't need to be pretty when you're up like that. You just need to find a way to kind of make those big moments, and we were able to do that." -- Islanders forward Brock Nelson
"We've got to look at it, but ... it's too easy to score against us right now. It's not because of our goalies. It's because we're losing races, we're not winning battles in front of the net, we're not boxing out. So whether it's tips or it's 2-on-1s or 3-on-2s, we're getting beat by just a couple feet." -- Capitals forward Lars Eller
Video: NYI@WSH: Eller jams home the Hagelin rebound
Need to know
The Islanders are 28-2-3 this season when they score at least three goals. ... Ovechkin was minus-4, tied for his worst rating of the season (Dec. 28 in 6-4 loss to Carolina Hurricanes). ... Samsonov was 2-0-0 with a 0.75 goals-against average and .970 save percentage against the Islanders. ... It was Carlson's 22nd multipoint game of the season. He leads NHL defensemen with 67 points (15 goals, 52 assists) in 56 games. ... Oshie has eight points (three goals, five assists) in the past eight games.
Islanders: Host the Philadelphia Flyers on Tuesday (Barclays Center; 7 p.m. ET; NBCSN, NBCSP, NHL.TV)
Capitals: At the Colorado Avalanche on Thursday (9 p.m. ET; ESPN+, ALT, NBCSWA, NHL.TV)
Video: Beauvillier scores twice, Islanders defeat Capitals