The Mariners' Stefen Romero had three hits and three RBIs, two on his fourth spring homer. Brad Miller also had three hits with two doubles. Mike Zunino hit a solo homer.
The Mariners improved their record to 17-10-3; the Royals are 11-14-2.
Hernandez said he originally was scheduled to throw six innings, but felt strong after five and 67 pitches and figured that was a good time to end his spring and start looking ahead.
"Now it's time to start a new season, and I'm ready for that," Hernandez said. "I saw everything I wanted to see. Every pitch was working really good. The first inning I was up a little bit and missed a couple pitches, but after that, I was fine. ... I feel really fresh. My arm feels good, my body feels good."
Hernandez was tagged for two runs in the first. Aoki and Billy Butler were on base with singles and two out when Alex Gordon rifled a double off the right-field wall. It rolled away from Romero, and by the time he chased it down, both runners were home.
After that, Hernandez retired 11 straight batters before Lorenzo Cain doubled with one out in the fifth and scored on Aoki's double down the left-field line.
Gordon hit a solo homer, his second of the spring, in the ninth inning.
Royals left-hander Bruce Chen lasted only four innings, giving up nine runs on 11 hits and two walks and running up 92 pitches in the process. He'll start the Royals' fifth game of the season.
Zunino, on his 23rd birthday, celebrated in the second inning by cranking Chen's first pitch high over the left-field bullpen for his second home run.
The assault on Chen continued in the third when Miller doubled and Romero blasted a home run over the right-center-field wall, his fourth. Before the inning was over, three more hits, two walks and an error by second baseman Pedro Ciriaco made it a six-run inning and an 8-2 lead for the Mariners.
Left-hander Francisley Bueno, trying to solidify his claim to the Royals' last bullpen slot, threw three scoreless innings. He gave up two hits but faced the minimum nine batters.
Royals first baseman Eric Hosmer was ejected by plate umpire Brian Hertzog after being called out on strikes in the third inning.
Up next: Right-hander Erasmo Ramirez makes his final spring start as the Mariners face the Rangers on Wednesday in a 12:05 p.m. PT game in Surprise. Ramirez, 23, has posted outstanding numbers (3-0, 0.96 ERA) in five Cactus League outings, but manager Lloyd McClendon wants him to work more on location on pitches when he's ahead in counts. Ramirez appears ticketed to pitch Seattle's second game of the regular season in Anaheim. The game will be available via a free audio webcast, exclusively on Mariners.com.