Oakland's victory, coupled with Minnesota's loss to Cincinnati, gave the AL West champion A's the No. 2 seed in the American League playoffs. They'll be home at the Coliseum for a best-of-three series against the seventh-seeded Chicago White Sox beginning Tuesday.
The A's owned the best record in the AL for much of the season before slumping down the stretch. They had lost five of seven before Sunday and were swept by the Mariners in a doubleheader Saturday.
"It's a clean slate going into the postseason," manager Bob Melvin said. "More than anything, it's been a difficult season on some guys. It's a new, clean slate. You can really make up a lot of ground by having a good postseason."
Oakland bounced back to win behind Montas' best start of the season and a steady day from the offense.
Montas, who will be in the bullpen for the first round of the playoffs, didn't get the win but was dominant in his final start. He gave up a pair of unearned runs, had at least one strikeout in every inning and struck out the side in the fourth and sixth. He allowed four hits and walked two.
"I went out there and just pitched," Montas said. "I didn't care about what happened."
Yusmeiro Petit (2-1) retired all three batters he faced to get the win.
Lamb's tiebreaking home run off Yoshihisa Hirano (0-1) leading off the seventh put the A's up 3-2. After Oakland loaded the bases later in the same inning, Canha hit a deep drive that deflected off the glove of Seattle center fielder Kyle Lewis as Sean Murphy and Tony Kemp scored.
Canha also had an RBI groundout off starter Marco Gonzales in the first and a bases-loaded walk in the eighth.
The A's will hold a workout at the Coliseum on Monday.
"The mood is always better after a win," Melvin said. "If you win six games in a row and you lost your last game, it's still a loss and there's still a different mood to it. I'm not going to say it isn't a better feeling going into the workout tomorrow."
Gonzales, who has not lost since Aug. 11, allowed two runs and six hits in five innings. He missed out on becoming the sixth pitcher in Mariners history to win six consecutive starts.
Even with the loss, Seattle made big strides in this shortened season. The Mariners, who were 68-94 and 39 games out of first place in 2019, finished 27-33, 10 games behind the A's.
After Sunday's loss, the Mariners held a team meeting.
"A number of players stood up and spoke, which is great. That's exactly where I hoped we would be as a team on the field, and it's exactly where I hoped we'd be as a team as far as a close-knit group in the clubhouse," manager Scott Servais said. "A lot of positives and we'll talk about it all offseason, but it's always disappointing when you go home and don't make the playoffs."
CLOSE THE DOOR
The start of the game was delayed briefly when an outfield door in center field swung open just before the first pitch. A's center fielder Ramon Laureano walked through the opening, closed the door and locked it, then climbed back over the fence and grabbed his glove.
Athletics: Chad Pinder had an RBI single in his return from the injured list and a strained right hamstring. Melvin watched him run the bases and was encouraged. "Having Pinder is huge for us," Melvin said. "We missed him." . RHP James Kaprielian was optioned to the alternate site.
Mariners: Servais and his club will take some time off and evaluate the roster before heading back to Arizona for spring training. Seattle plays its first game in Peoria against the San Diego Padres on Feb. 27.
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