It was the Thrashers' first shutout of the season. For Lehtonen, it was his 12th career shutout and first against the Devils.
Kovalchuk, added Thursday to the Eastern Conference team for the NHL All-Star game, lived up to the billing with key contributions in Atlanta's three-goal second period.
Lehtonen also had a star turn to preserve the win, coming up with a big pad stop on Brian Rolston's breakaway bid in the scoreless first period.
New Jersey, which had won five of the previous six games against Atlanta, suffered its fourth shutout of the season, all at home.
Armstrong opened the scoring at 3:03 of the second, chipping a shot into the slot that dribbled through Devils goalie Scott Clemmensen.
Kovalchuk upped the lead to 2-0 with a power-play blast from the high slot at 5:31. He later nudged the puck ahead to create the 2-on-1 break for Slater and Chris Thorburn that Slater polished off at the 14-minute mark.
Clemmensen, making his 13th straight start for the Devils, was yanked after allowing three goals on 19 shots through two periods.
Kevin Weekes took over in the third, and was victimized on the first shot he faced when Reasoner made it 4-0 at 2:09.
Notes: The Devils head out on a six-game trip that starts in Los Angeles against the Kings on Saturday. They return on Jan. 21 to host Montreal before playing two more on the road in Ottawa and Boston. ... Devils D Colin White returned after missing five games with an upper-body injury. With White back, the Devils scratched Jay Leach. ... Each team had a forward named Thursday as a reserve for the All-Star game: Kovalchuk for the Thrashers and Zach Parise for the Devils. ... C Bryan Little, the Thrashers' leading goal scorer with 19, missed his second straight game because of a rib injury.
