A's starter Jarrod Parker even committed what seemed to be a costly error -- a misfired throw to first base on Dustin Ackley's sacrifice bunt that resulted in runners at the corners with no outs and the game tied, 1-1, in the eighth inning.
But Parker battled back, striking out Humberto Quintero swinging, forcing a pop up to Brad Miller in foul territory and striking out Nick Franklin, who promptly reacted by throwing away his equipment in disgust.
Oakland capitalized with Moss' second career walk-off home run of the season, belting a 2-and-1, 96-mph fastball delivered by Carter Capps.
"I've watched it about four for five times already and I felt like I executed the pitch pretty good," Capps said. "I was trying to throw a strike low and away, and I did. He just got through it."
"I was very surprised that went out, because this place this year, at night, I know we've hit balls well at night that just don't go like they did last year," Moss said. "I was very happy when that ball went over the fence, because when I hit it, I was worried, not gonna lie."
The home run spoiled Aaron Harang's best outing in a month, as Parker bested him with his first career complete game on just 100 pitches.
Harang gave up five hits and a walk with three strikeouts in seven innings. His lone run allowed was the result of an RBI groundout by Josh Donaldson with runners at the corners in the fourth.
The Mariner's right-hander credited his strong outing to an ability to mix in his curveball and changeup effectively, an encouraging carryover from his last outing against Tampa Bay that ended a stretch of poor starts.
"When you can throw it for strikes and also throw it for a put-away pitch, it makes them have something else to worry about," Harang said. "When you can throw a couple different pitches around the zone and get strikes, and also throw chase pitches, it makes it a lot harder for them to start guessing and just sit on one pitch.
Seattle's run came in the seventh when Kendrys Morales singled in Franklin, but it wasn't enough. The Mariners might have scored more if Morales hadn't been thrown out at third by A's right fielder Josh Reddick to end the inning.
Morales appeared to prematurely slow down short of the bag to leave just a wide enough opening for Reddick -- last year's Gold Glove Award winner -- to fire a strike that nailed Morales for the third out of the inning.
"I think he was assuming that Reddick was going to go to second base, and that's one guy you don't really want to challenge, especially if you're kind of a below-average runner, he's going to try and get you," Mariners active manager Robby Thompson said.
"I didn't know he'd have a chance to throw to third," Morales said through an interpreter. "I was surprised about it."