Seattle added another run in the seventh when Ben Gamel singled home Gordon Beckham. The Mariners got their first two runs via Alonso's 27th homer in the fourth.
The A's took the lead in the sixth when Khris Davis hit a two-run homer to right-center off Seattle starter James Paxton. The homer was Davis' 42nd, matching his career high from last season.
Davis' blast knocked Paxton out of the game, the only other damage against him being a leadoff homer by A's shortstop Marcus Semien. The left-hander allowed three runs on six hits and struck out five in 5 1/3 innings.