Saunders (5-6) was touched for just one run on five hits and struck out four to earn his second win in three starts.
Mike Zunino hit a solo homer and Raul Ibanez had two hits and an RBI for the Mariners, winners in three of their last four games.
Oliver Perez stranded a runner in the ninth to nail down the first save of his career.
Chris Young went 3-for-3 with a solo homer and Eric Sogard added an RBI for the Athletics, who had an 11-game home winning streak snapped.
Tommy Milone (6-6) was charged with two runs on five hits over 6 2/3 innings to absorb his first loss since falling to Seattle on May 12.
Seattle took a 1-0 lead in the second inning. Michael Morse opened frame with a double and moved to third on an Ibanez base hit. Zunino then grounded into a 6-4-3 double play, but Morse was able to score on the play.
Oakland looked to knot things up in the home half. With a runner on second, Derek Norris ripped a single to center field. Nate Freiman had stopped at third base, but after Michael Saunders bobbled the ball in center, broke for home. Saunders was able to recover, though, and delivered a strike to catch Freiman at the plate.
The A's tied the game in the fifth inning on Sogard's double. With two outs, Sogard popped up to shallow right field. M's second baseman Nick Franklin ran back to field the ball, but collided with right fielder Jason Bay and the ball skipped away allowing Adam Rosales to score from first base.
Zunino clobbered his first career homer in the seventh inning to put Seattle in front for good. Milone left a changeup over the heart of the plate and Zunino drove it over the wall in center field.
"That ball went a long way," Seattle manager Eric Wedge said. "It went up in the air different, so you knew it had a chance to end different, and that's what happened."
With runners on the corners in the eighth inning, Ibanez roped a line drive down the first base line. Freiman made a diving attempt at the ball, but couldn't field it cleanly, and Franklin crossed the plate to give Seattle an insurance run it would need.
Young led off the bottom of the eighth with a towering homer into the second deck in left field.
"I felt good today," said Young. "No matter where I'm at in the lineup, I'm trying to produce and make the necessary adjustments to have some success. That definitely sparked me up a little bit."
Josh Reddick laced a two-out single to right field in the ninth inning, but Perez got Rosales to fly out to right to end the game.
The Mariners' starting pitchers have recorded a quality start is each of their last seven games ... Seattle was just 2-for-11 with runners in scoring position ... Oakland's last setback at home came on May 14 when the A's fell to the Texas Rangers, 6-5, in 10 innings ... Oakland had a three-game winning streak snapped.
06/15 03:28:58 ET