The A's clinched the American League West division title and secured a spot in the post-season with their big win after fellow contenders for the west the Los Angeles Angels went down 5-2 to the Texas Rangers.
Oakland's Milton Bradley hit a three-run homer in a four-run second innings as the A's snapped a three-game losing streak and wrapped up their 14th division crown - the third most division titles in the major leagues behind the Atlanta Braves (16) and the New York Yankees (15).
Starting pitcher Rich Harden allowed just two hits in five scoreless innings, with the Athletics winning eight from eight in games started by the right-hander this season.
For the Mariners, Ichiro Suzuki was not in the lineup, ending his club-record streak of 396 consecutive games played.
The A's join the Yankees and the New York Mets in the playoffs with the race for the final five places hotting up.
In the American League Central, the Detroit Tigers and Minnesota Twins both had narrow victories over the Toronto Blue Jays and Kansas City Royals respectively, with the Tigers holding a one-win break over the Twins at the top of the division with five matches remaining.
Both the Tigers and the Twins will play in the post-season - one as American League Central division winners and one as the American League wildcard team.
National League spots up for grabs
In the National League Central division, the Houston Astros' 7-4 win over the Pittsburgh Pirates allowed them to close in on current division leaders, the St Louis Cardinals.
The Astros are now just 1.5 wins behind the Cardinals who have lost their past seven straight games in contrast to Houston's past seven straight wins in what should be a great finish to the NL Central race with the Cincinnati Reds also in contention 2.5 wins behind.
The National League West division is down to a two-horse race with the San Diego Padres Leading the Los Angeles Dodgers by two games after the Dodgers destroyed the Colorado Rockies 11-4.
The Padres finish their season with a four match series away to the Arizona Diamondbacks, while the Dodgers have two more matches in Colorado, and then three matches in San Francisco against the Giants.