San Jose (44-26-10), which clinched a playoff berth Monday, lost its fourth straight game (0-3-1) and remained two points ahead of the Los Angeles Kings and three ahead of the Anaheim Ducks. The Sharks and Kings each have two games remaining, and the Ducks have three.
"We got to find a way to win," said San Jose forward Logan Couture, who scored in the first period. "We're fighting for home ice in the first round here. We have to dial it up. You can't coast for very long in the NHL and win hockey games."
Lehtonen allowed two goals on 14 shots before leaving the game at 15:54 of the first period with an upper-body injury. McKenna made 17 saves in his first NHL game since Feb. 16, 2015, with the Arizona Coyotes.
"It's one of those things where I've been up (in the NHL) but never had the minutes," McKenna said. "You just have to stay diligent and work hard in practice. It is fulfilling getting a win. It was fun to be a part of the win. Sometimes you lean on your experience. It's not anything I haven't seen before. It turned out to be a good night for everybody in the room."
Dallas coach Ken Hitchcock said Lehtonen should be healthy enough to play against the Ducks on Friday, but McKenna probably will start.
"It's a little early, but sometimes a guy just earns it, and quite frankly, he earned it," Hitchcock said.
Couture gave the Sharks a 1-0 lead at 9:21 of the first with his NHL career-high 33rd goal, scoring from above the left face-off circle on the power play.
Timo Meier made it 2-0 at 15:16 with his NHL career-high 20th goal. He beat Lehtonen when he quickly spun and took a wrist shot from below the left circle after getting to a loose puck.
"We got a good lesson in not putting a team away when you get a chance to," Sharks coach Peter DeBoer said. "We got up 2-0 in the first. I thought the second period we played better than the first and had a chance to really take control of the game.
"You have to give their goalie some credit. He made some saves, but you let a good team hang around like that with three world-class players like they've got, that's what can happen. So, tough lesson to learn this time of year, but we still control our own destiny and get back up tomorrow and go back at it."
After Benn got the Stars within one late in the second with his 30th goal, Gemel Smith tied it 2-2 at 14:43 of the third when he put the rebound of a Devin Shore shot past Jones (19 saves) from below the right circle.
