"If I'm able to go out there and throw 40 saves, there's a great chance the Braves are going to the playoffs," he said in his first day with the team.
As it turned out, 37 was enough to help the Braves make their first postseason since 2005.
Wagner and the Braves will face San Francisco in an NL division series beginning Thursday.
Wagner struck out the side in the ninth inning for the save in Sunday's 8-7 win over Philadelphia as the Braves earned the NL wild card.
Manager Bobby Cox's retirement after the season has been the big story in Atlanta. The 38-year-old Wagner also is calling it quits after the season, and he already has a memorable final regular-season game.
Wagner notched his 422nd save, fifth on the career list and only two behind John Franco's record 424 for left-handers. It wasn't easy, because a rocky eighth inning preceded the dominant ninth.
The Phillies scored three runs off Jonny Venters and Wagner in the eighth. After allowing an RBI single by Wilson Valdez and pinch-hitter Ben Francisco's two-run double, Wagner struck out Raul Ibanez to strand runners on second and third.
Wagner realized how close he came to losing the lead and blowing more than the save. A loss could have left a bitter final memory for his career.
"Don't think that didn't run through my head out there. You screw this up and you have to go home and always be a goat," Wagner said. "I was happy to go out there and compete and make some pitches and have another day to go out there and sweat."
After winning the key showdown with Ibanez, Wagner had more strikeout magic in the ninth. He struck out Shane Victorino, Brian Schneider and Greg Dobbs -- each on called third strikes.
"What he did in the ninth was amazing," Braves catcher Brian McCann said. "That's why he's had so many saves and he's one of the best closers of all time."
Cox said Wagner "has been on me all year" to come in a game in the eighth.
After the eighth-inning struggle, Cox said he never wavered in also giving his closer the ball in the ninth.
Cox joked that if he'd turned to another reliever, "you'd have been given a new manager. I'd be dead."
This is Wagner's eighth postseason. Wagner and the Houston Astros lost to the Braves in the division series in 1997, 1999 and 2001. Wagner and the New York Mets beat the Dodgers in the 2006 division series before losing to St. Louis in the National League Championship Series.
Wagner's 37 saves have come in 44 chances.
He begged Cox all season for a chance to pitch more than one inning and finally had his chance with the season on the line.
"I wish I had made it a little bit easier," he said. "They made me work my tail off. I was lucky to get through that eighth."
Wagner insists he is committed to spending more time with his family in Virginia and will stand by the retirement announcement he made early in the season.
"This is it for me," Wagner said Sunday. "I'm gone." He said having one more chance for his first World Series "makes it that much more special."