McCarthy (8-7) pitched his second career shutout, hurling 78 of his 114 pitches for strikes for the A's, who clinched their three-game series with the Mariners.
The right-hander allowed only two runners to reach as far as second base and didn't issue a walk.
Michael Pineda (9-9) absorbed the loss for Seattle, yielding three runs on five hits and two walks over six frames. He fanned seven.
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