Seattle built a 5-1 lead after two innings and never trailed.
Five players had two hits, including Raul Ibanez, who drove in six runs in the series with six hits in all.
Ichiro Suzuki had two RBI and extended his hitting streak to 12 games with a single, as the Mariners banged out 16 hits in their first three-game sweep of the A’s in Oakland in six years.
Winner Felix Hernandez (13-7) beat Oakland for the fourth time this season.
Four players had one RBI, including Jack Cust, who hit his 24th homer in the fifth. Oakland scored one run on Hernandez's second-inning throwing error.
Jack Hannahan and Jeff DaVanon had two hits apiece, but the Athletics' losing streak increased to four straight games.
The loser was Dan Meyer (0-2).
Seattle trails the LA Angels of Anaheim by 8 ½ games in the AL West. LA can clinch the division with any combination of Angels' wins or Mariners' losses totaling three.
Notable deaths of 2014 [PHOTOS]