Hernandez (2-0) retired the first 11 batters on the way to his 16th career win over the Athletics, his most against any team. Hernandez walked one and struck out eight while allowing a run on six hits over 8 1/3 innings.
Dan Straily (0-1) allowed three runs on six hits over his six innings. He walked one and struck out seven.
Straily carried a shutout into the fifth inning before Ackley's towering two-run shot gave the Mariners a 2-0 lead. With two outs, Almonte drove Straily's 1-0 pitch even deeper into the right field bleachers.
The way Hernandez was pitching, the run support was more than enough.
Jed Lowrie, who hit a home run leading off the ninth, broke up Hernandez's no-hit bid with a two-out single in the fourth.
Hernandez improved his record to 67-3 when going at least seven innings and allowing three or fewer runs.
The Mariners pitching staff extended their club record season-opening streak to allowing three or fewer runs to five.
Hernandez, who turns 28 on Tuesday, ranks fourth on the all-time list with 1,722 strikeouts before the age of 28 behind Sam McDowell, Bert Blyleven and Walter Johnson.
NOTES: RHP Sonny Gray (0-0, 0.00) will start Sunday's series finale against the Mariners. He's working on a 13-inning scoreless streak dating to last year after pitching six innings in the season opener. . Erasmo Ramnirez (1-0, 2.57) starts for the Mariners. He gave up two runs on six hits over seven innings in his first start. . Cano is batting .342 (27 for 79) in his last 19 games in Oakland. . The A's first four games averaged 3 hours, 29 minutes. . Hernandez is 5-0 in his last 10 games in Oakland. . Cespedes, who has hit 10 homers against Seattle pitching, hit a ball into the left field bleachers in the ninth that curved foul a pitch before flying out to right.