Seager went 4-for-5 with a three-run homer, two triples and three runs scored, with Saunders coming through with a solo shot and a two-run single in Seattle's fifth consecutive road win over New York. Two of those victories came around an April 30 rainout that was made up on Monday.
Brad Miller added a two-run single that snapped a 2-2 tie in the seventh inning, while Mike Zunino finished 2-for-4 with an RBI in the rout.
Hernandez (8-1) aided the cause by continuing his career-long dominance in the Bronx. The Mariners' ace struck out eight without a walk while allowing just two runs over seven innings, improving to 5-1 with a 1.37 ERA in seven career assignments at the current Yankee Stadium.
Six of Seattle's runs were charged to David Phelps (1-3) in the Yankee starter's six-plus inning stint.
Former Mariner great Ichiro Suzuki knocked in New York's lone two runs with a fourth-inning single. Yangervis Solarte ended 2-for-4 with a run scored in the loss, the Yankees' third in four games and 10th in 14 outings at home.