Mason Marchment had a goal and an assist, and Radek Faksa, Jason Robertson and Jani Hakanpaa also scored to help Dallas win its second straight game and increase its lead to three points on Colorado and Winnipeg in the Central Division.
Benn and Seguin were reunited on the same line against the Islanders after spending most of their first nine seasons together in Dallas as linemates.
“They kind of got out there on a line change, and made the first goal happen,” Stars coach Pete DeBoer said. “I decided to keep them together. That line was great tonight.”
Jake Oettinger stopped 27 shots to improve to 7-2-1.
“We didn’t finish strong enough,” Islanders coach Lane Lambert said. “Defensively in our zone, we had a couple of breakdowns, and when we broke down they took advantage of us.”
Benn gave the Stars a 3-2 lead at 8:17 of the third when he got a pass form Marchment from behind the net and flipped a backhander up high to beat Varlamov for his eighth.
“He's playing fantastic,” DeBoer said.
The Islanders had a chance to tie the game with four minutes to play and Oettinger lying on the ice outside the crease, but defenseman Ryan Suter scooped the puck away with his stick before it could cross the goal line.
Robertson whirled clockwise at the top of the right faceoff circle and whipped the puck inside the left goalpost to give Dallas a two-goal lead with 3 1/2 minutes remaining. Hakanpaa sealed the win with an empty-netter with 21 seconds left.
DeBoer wouldn't say whether he would keep Benn and Seguin together.
“It's tough to ignore that chemistry and that success that they had tonight,” he said.
Benn said he enjoyed the linemate reunion.
“It's fun. We've been together for 10 years now." Seguin said, “It feels good. ... That was a gutsy win tonight. It was just another way to win. We've opened it up a little bit. In the past, it was a grind game.”
Barzal got the Islanders on the scoreboard first at 1:30 of the second, tipping Alexander Romanov's shot from the left circle past Oettinger for his first of the season.
Faksa tied it at 6:57 when he got a pass from Seguin and snapped a shot past Varlamov from in front for his second.
Barzal's second goal, on the power play, followed a pass from Brock Nelson to Barzal in the slot to give the Islanders a 2-1 lead with 5:41 left in the middle period.
Just 40 seconds later, Benn set up Marchment in the slot to beat Varlamov with a wrist shot to tie it 2-2.
Scott Wedgewood returned as the backup goalie for the Stars two days after he was taken off the ice on a stretcher. ... Roope Hintz, who scored two goals on Thursday, was a late scratch Saturday with a lower-body injury.
Barzal ended a 22-game drought, dating back to April 23 against Buffalo. ... Seguin has 309 goals and 392 assists. He is the first NHL player from the 2010 draft to reach 700 points. “I feel pretty young. Two games ago, I realized where I was, There's never been an internal goal where to finish,” Seguin said. ... Islanders C Kyle Palmieri played in his 700th NHL game and D Adam Pelech skated in his 400th. ... Robertson extended his points streak to 11 games.
Islanders: At Toronto on Monday night to complete a four-game trip.
Stars: Host Colorado on Monday night in the second of a four-game homestand.
Lary Bump, The Associated Press