Padres first baseman Mitch Moreland plated the go-ahead run in the 11th inning of the 7-4 victory Sunday at Petco Park in San Diego. The Padres haven't made the playoffs since 2006, the third-longest drought in Major League Baseball.
San Diego had nine consecutive losing seasons before the 2020 season.
"For us to be able to break that drought and clinch a spot in the postseason is huge," Padres designated hitter Eric Hosmer told reporters. "It's one thing to talk about it, but to finally feel it and go out and do it is a great accomplishment.
"We have a long way to go, but we are going to enjoy this because this is a big step for us."
The Padres (34-20) are on track to clinch the No. 4 seed in the National League and face the No. 5 Miami Marlins in a wild card series. Miami hasn't made the playoffs since 2003, MLB's second-longest drought.
San Diego is in second place in the National League West behind the Los Angeles Dodgers, who have the best record in baseball. The Padres, who have won seven of their last 10 games, have the second-best record in the National League and fourth-best in Major League Baseball.
Padres pitcher Dinelson Lamet allowed just two hits and one run in six innings, but was not on the record for a victory in Sunday's win.
"It's a great moment," Lamet said. "This is something that hasn't happened for a long time here. It's something we've been wanting to get to. There is a lot of pride and it's a privilege to get to the postseason."
Tim Lopes gave the Mariners a 1-0 lead with a double in the fifth inning and Wil Myers hit a three-run homer in the sixth to give the Padres a 3-1 lead.
Mariners second baseman Dylan Moore responded with a two-run homer in the bottom of the eighth frame to tie the score at 3-3. Both bullpens held up in the ninth to force extra innings.
Padres slugger Fernando Tatis Jr. hit an RBI single in the top of the 10th inning for a 4-3 lead. The Mariners responded with a J.P. Crawford RBI sacrifice fly in the bottom of the inning to tie the score.
The Padres started the 11th inning with Manny Machado at second base. Moreland smacked a Casey Sadler 0-1 sinker to left field to plate Machado for a 5-4 lead. Myers struck out in the next at-bat before Jake Cronenworth drew a walk.
The Mariners then got the second out of the inning with a force out. Left fielder Jurickson Profar followed with an RBI single. The Padres then went up 7-4 when Austin Nola scored on a fielding error.
Padres relief pitcher Trevor Rosenthal retired the Mariners' final three hitters to seal the victory.
"It has been a quick, but hard year," Tatis said. "Everybody has struggled and put in good work. We have each other's backs and find a way to win games."
The Padres next host the Los Angeles Angels at 9:10 p.m. EDT Tuesday at Petco Park. They have six games remaining in the regular season.