Scott Mellanby and Brad Larsen also scored for the Thrashers, who snapped the Maple Leafs' eight-game winning streak in the series by handing Toronto its first shutout of the season.
It was Atlanta's first win over the Maple Leafs since a 3-2 overtime victory on Oct. 27, 2003, in Toronto. The Maple Leafs outscored the Thrashers 37-9 in their eight straight wins, including a 4-2 victory on Oct. 30 in Toronto.
Hossa ended his personal history of frustration against Toronto with his first hat trick for Atlanta. Hossa leads the NHL with 20 goals this season, but he had only one goal and one assist in his last nine games against Toronto, a streak that began before Hossa's 2005 trade to Atlanta from Ottawa.
Hossa passed the New York Rangers' Jaromir Jagr for the league scoring lead with 39 points. Hossa also leads the league with 20 power-play points.
Toronto lost its third straight, matching its longest skid of the season. It scored only one goal in each of its back-to-back losses to Boston before facing more scoring problems against Lehtonen and the Thrashers.
Lehtonen earned his first career win in three games against Toronto. He recorded his fourth shutout of the season.
Hossa's first goal gave the Thrashers a 1-0 lead with 6:52 left in the first period. Hossa waited about five feet to Andrew Raycroft's left as the Thrashers crisply executed passes from Slava Kozlov to Mellanby to Hossa, who was left with a clear opening for his short wrist shot.
Raycroft kept Atlanta's lead at 1-0 as he stopped breakaway attempts by Hossa in the first period and by Shane Hnidy early in the second.
Hossa and the Thrashers pushed the lead to 2-0 less than two minutes into the third period. Racing down the ice, Hossa took a pass from Kozlov with a skate, kicked it up to his stick and then scored on a snap shot in front of the net.
Jon Sim was charged with slashing midway through the third period, setting up a power play and two goals within 14 seconds for the Thrashers.
Mellanby scored the first on a pass from Hossa, and Larsen pushed the lead to 4-0 seconds later when he scored off Bobby Holik's assist.
Atlanta fans were calling for Hossa's hat trick when his third goal capped the scoring with about three minutes left.
Notes: The Thrashers activated defenceman Steve McCarthy from injured reserve. McCarthy missed eight games with a shoulder injury. ... Toronto D Wade Belak drew a four-minute penalty for high-sticking in the second period after sending Sim off the ice with an apparent hand injury. Sim returned. ... Mellanby scored his first goal in 14 games. ... The Thrashers improved to 9-0-1 when allowing no more than two goals.