Merrifield's homer came off Padres lefty Drew Pomeranz.
"I have faced Drew before," Merrifield said. "I'm pretty familiar with what he's got. For some reason, I've always seen the ball pretty good off him. I got a good count, got ahead 3-1 and put a good swing on it."
Merrifield, who can play the infield and outfield, has had an impressive spring, hitting .395 with three doubles, two triples and two homers.
"He's had a real good spring," Royals manager Ned Yost said. "I would feel comfortable putting him at any position in a big league game."
If nothing else, Merrifield is making the Royals' decisions on the final 25-man roster very difficult.
"I am just trying to do what I can to show them what I can do," Merrifield said. "That's the only thing I can control. Whatever they decide, they decide."
Jeffrey Flanagan is a reporter for MLB.com. Follow him on Twitter at @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.