Dainius Zubrus also scored for New Jersey (21-30-4), which improved to 11-1-2 in its past 14 games.
Nikolai Kulemin scored for the Maple Leafs (23-26-6), who moved within seven points of Carolina for the final playoff spot in the East. The Hurricanes lost 2-1 to Philadelphia on Thursday.
The Maple Leafs wasted an excellent outing by James Reimer. It was hardly the fault of Toronto's rookie goaltender, who finished with 36 saves and was responsible for the game getting past regulation.
Kovalchuk's winner came at the end of a nice solo rush where he took a shot that sneaked behind Reimer helping Lemaire hit the milestone.
Scotty Bowman, Al Arbour, Dick Irvin Sr., Pat Quinn, Mike Keenan are also among the coaches also in the 600-club.
Joffrey Lupul had plenty of jump one day after being acquired by the Maple Leafs in a trade from Anaheim, but saw the puck bounce over his stick on a 2-on-1 break in the third period. That was his best scoring chance.
As has often been the case of late, Toronto was let down by its power play. The team went 0 for 5 against the Devils and is 3-for-40 during the past 11 games.
Toronto led 1-0 after 40 minutes but Zubrus tied it up 3:23 into the final period. He redirected a Mark Fayne point shot over Reimer's shoulder for his 11th goal of the season.
But the Leafs' goalie was strong from there, denying Zubrus with his glove from in close before getting in front of a Kovalchuk one-timer on the power play. Reimer also got his shoulder on Fayne shot through traffic with about four minutes left in regulation.
Toronto's Phil Kessel was denied on a couple chances just before setting up Kulemin's 21st goal of the season at 5:18 of the second period. The winger managed to slide the puck in front while being bodied by Devils defenseman Colin White, and Kulemin had a wide open net.
New Jersey had a chance to tie it during a 4-on-3 power play, but Reimer came out and stopped a Brian Rolston shot with his blocker.
Rookie defenseman Keith Aulie was also in Toronto's lineup after being called up earlier in the day. He took shifts on the top pairing with Dion Phaneuf - essentially replacing the departed Francois Beauchemin - and was a physical presence.
Notes: Devils defenseman Anton Volchenkov sat out the first game of his three-game suspension ... Kessel hasn't scored in 13 games. His career-worst drought lasted 15 games - it happened twice during his rookie season in 2006-07 .