Victor Hedman tied the game 1-1 at 18:25 of the first with a slap shot from the left point that went past a screen from Barclay Goodrow and off the left post. Hedman has scored a goal in four straight games.
"Up until that point, [the Islanders] pushed hard in the first," Hedman said. "We were a little bit on our heels, but we got that goal. Nothing fancy about it. We win a face-off and go to the net. The puck obviously had eyes, but we'll take it."
The Islanders had a 5-on-3 power play for 38 seconds in the third period after McDonagh was called for holding at 7:48 and Cedric Paquette was called for hooking at 9:10.
"When you have a 5-on-3, you want to capitalize," New York forward Brock Nelson said. "You have like 38 seconds, so plenty of time. You're hoping to get a look there. That can be a difference maker in the third."
Lightning forward Brayden Point left the game for undisclosed reasons at 7:14 of the second period after finishing his last shift at 5:23. Tampa Bay coach Jon Cooper did not have an update following the game.
Point scored five points (two goals, three assists) in Game 1, and his 23 points (eight goals, 15 assists) are second in the NHL in the postseason behind Colorado Avalanche forward Nathan MacKinnon's 25.
Nelson returned to the New York bench as Point exited. Nelson left the game after he was hit into the boards in the first period by Tampa Bay forward Alex Killorn, who received a game misconduct and a five-minute major for boarding at 5:55.
Without Point and Killorn, and using seven defensemen, the Lightning played with nine forwards.
"Rarely do you lose guys in a game, let alone two guys, early," Cooper said. "[Killorn] basically didn't play the entire game. Point maybe played a shift or two more. So that's a long stretch to go with just nine.
"It's a gutty, gutty effort. … In the end, it was gutty. That's what that effort was. It was just gutty."
NOTES: Lightning forward Ondrej Palat's five-game goal streak ended. He scored six goals during the streak, which was the longest in Lightning playoff history. … Islanders forward Cal Clutterbuck had three shots on goal in 10:27 after leaving Game 1 at 11:36 of the second period after he blocked a shot. … Nelson said he felt fine after being seen by Islanders doctors and returning to the game.
NHL.com staff writer Mike Zeisberger contributed to this report