Slava Kozlov added a goal and two assists for the Thrashers, who lost a 3-2 shootout to Tampa Bay in their season opener on Thursday night.
"The first game we played good in the first period, but we did not in the other two," Hossa said of the opener in which the Thrashers scored two first-period goals then went scoreless.
"Tonight we knew we had to play a full 60 minutes. We did everything right tonight and Kari was awesome."
Lehtonen, in his second season, earned his fourth career shutout.
"That's always great (getting a shutout)," he said. "The hard thing is to get this out of my head and focus on the next game."
Atlanta opened the scoring on a goal by Jon Sim only 6:54 into the game, beating Florida goalie Ed Belfour on a 30-foot blast from the right circle. Bobby Holik made it 2-0 on short-handed goal when he put a shot between Belfour's legs at the 13:39 mark.
Hossa and Kozlov then took over the scoring in the second period.
Kozlov scored front up close on a perfect pass from Hossa only 1:33 into the period. Hossa then made it 4-0 on a power play at 6:45 when he tipped in a nice pass from Ilya Kovalchuk in front of the net.
Panthers coach Jacques Martin replaced Belfour with Alex Auld, who was in goal for Florida's 8-3 season-opening victory over Boston on Friday night. Belfour had only five saves against the Thrashers, missing both shots in the second period before being lifted.
"I have no excuses. I just have to play better," said Belfour, a 41-year-old free agent who played with Toronto last season. "I thought I had a good warmup, but obviously that didn't carry over to the game."
Hossa then added his third goal of the season and second of the night on another power play with a wraparound goal, sneaking it past Auld at 14:27.
Andy Sutton scored 5:44 into the third period to give the Thrashers a 6-0 lead.
Lehtonen, limited by injuries to only 39 games last season as a rookie, made his toughest save with an outstanding stop on a shot from Todd Bertuzzi at point-blank range in the opening period. He's 6-1 against Florida with two shutouts.
Atlanta, which led the league with 58 power-play goals last season, had two against Florida.
- Hossa has five points in two games.
- The Thrashers reassigned goalie Fred Braithwaite to their Chicago farm club of the American Hockey League prior to the game.
- The Thrashers, who matched a team-low 11 shots on goal in their opener, had 14 shots in the second period alone and finished with 34.