Tom Wilson and Alex Ovechkin scored for the Capitals (21-8-9), who were 0-2-2 in their previous four games.
"We had a little disconnect the last couple of games," Wilson said. "[The Islanders] have a heavy forecheck, so we were forced to come back in the zone, help our [defensemen] break it out, break out with speed, get our feet moving.
"We were rolling them over for the first 40 minutes. I think they had a bit of a wave in the third where they pushed a little bit, but that's expected. We weathered it for two or three shifts and got back to work. We've got to take the two points and keep building on that."
Semyon Varlamov made 34 saves for the Islanders (11-13-6), who were shut out for the fourth time this season. New York had won three in row and was 4-0-1 in its past five games.
"It felt like a game where we had a layoff," Islanders associate coach Lane Lambert said. "I was expecting more of a lack of execution potentially last game (a 3-2 win against the New Jersey Devils on Thursday) than this game. But certainly, we knew the magnitude of the game. We didn't get the result."