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.
More Sports Headlines
Wambach remembers loss to Japan in World Cup final in 2011 Montgomery solid into 6th, Mariners beat Athletics 2-1 4:02 p.m. Rain helps Hamilton win thrilling British GP, extend F1 lead 3:56 p.m. Lee wins Greenbrier Classic in 4-man playoff 3:54 p.m. Froome and Contador in good positions in Tour de France 3:48 p.m. All-Williams matchup headlines Monday's action at Wimbledon 3:37 p.m. AquaSox have cooled off in the heat Cedarcrest Golf Course cuts back on watering the course
Our new comment system is not supported in IE 7. Please upgrade your browser here.