Hernandez, who signed a seven-year $175 million deal last month, struck out two and threw 33 pitches.
Romero, an infielder, finished 4-for-5. He had a grand slam in the fourth inning and a three-run shot in the ninth.
Casper Wells hit a three-run homer in the eighth. The Mariners have 10 consecutive multi-homer games and top the majors with 29 home runs.
Hisashi Iwakuma allowed one hit in three scoreless innings for the victory.
Royals starter Ervin Santana struck out four in three innings and allowed a run and three hits.
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