Mellanby, a 20-year veteran who is nearing the end of his career, scored a power-play goal at 6:27 of the third period to snap a tie and help the Thrashers beat the Florida Panthers 5-2 Wednesday night.
"To be able to have the kind of night I had and have a big goal like that is special for me," said Mellanby, who is considering retirement. "It was gratifying in a lot of ways."
The Thrashers moved within five points of eighth-place Tampa Bay in the Eastern Conference. Both teams have seven games to play. Florida is eight points behind the playoff cutoff with six games left.
Atlanta and Tampa Bay will face each other Thursday and April 11, both times in Tampa.
"At least with tonight's game, it's setting up for the two games with Tampa Bay," Thrashers coach Bob Hartley said. "At least we gave ourselves a chance."
Marian Hossa followed Mellanby with two third-period goals, including an empty-netter with 22.9 seconds left.
Kari Lehtonen, making his 27th consecutive start for the Thrashers, stopped 38 shots.
Roberto Luongo made 29 saves and remained in a tie with John Vanbiesbrouck for the most career victories by a Florida goaltender (109).
"We didn't have a whole lot of hope left, but there was still a little bit," Luongo said. "Obviously, after tonight it would take a major miracle, so there's nothing we can do about it but keep fighting and play hard the rest of the year."
Mellanby gave Atlanta a 3-2 lead at 6:27 when he put in a loose puck with Luongo down on the ice. Mellanby tipped Kovalchuk's slap shot from the point off the post, and Luongo stopped Marc Savard's rebound attempt before Mellanby skated in.
"He was just being the Scott Mellanby that started with us at the start of the year," Hartley said. "He's a great competitor. He's at the net, he's paying the price."
"He's such a powerful skater," Hartley said. "There's not too many defensemen who can keep up with him. It's not surprising. He's not impressing me with those plays anymore. The guy's a force of nature."
The teams traded four goals after a scoreless first period, with Florida twice taking the lead.
Kolnik opened the scoring at 1:44 with a power-play goal, scored when he put in a rebound from the side of the net.
McCarthy tied it at 6:35 when he grabbed a loose puck at the side of the net after Luongo come out of the crease to make a save.
After Florida killed a 4-minute power play, Gelinas gave the Panthers the lead again when he put in a rebound.
Kovalchuk scored his 49th of the season at 14:53 during a power play. He got to a rebound at the side of the net and scored.
"It was a huge game for us," said Florida center Chris Gratton, who was in the penalty box when Mellanby scored. "That's a team we could have leapfrogged, but they played a great game. They took the play to us. The first period and a half we played our style but then they took over. It's a devastating blow, that's for sure."
- Atlanta set a franchise record with its 17th road victory of the season. The previous mark was set in 2002-03.
- Florida's Jokinen was helped off the ice near the end of the first period when he injured his right leg while crashing into the boards. He returned for the start of the second period.
- With his second-period assist, Jokinen became the first Florida player other than Pavel Bure to reach 80 points in a season.
- Atlanta leads the season series 5-2 after recording the first victory by a visiting team. The clubs will meet in the season finale on April 18 at Florida.