Padres shortstop Andy Parrino launched his fifth homer of the spring.
Seattle struck first against highly regarded Padres prospect Joe Wieland. Center fielder Denny Almonte's infield hit leading off the third inning was followed by Miller's triple, which seemed to fool center fielder Cameron Maybin. Poythress lined his second single, to center, to cash in Miller.
Millwood (3-1) kept the Padres off the board until Parrino unloaded over the left-field wall with one out in the fifth. Millwood had allowed only two hits and two walks to that point.
The Padres were thwarted in an attempt to draw even in the sixth, a double-play grounder by Yonder Alonso (who had singled and doubled) sandwiched between Will Venable's single and Nick Hundley's double.
After Wieland (0-2) departed with five solid innings (two earned runs, five hits, a walk and five strikeouts), Anthony Bass took over, and the Mariners scored twice.
Stolen bases by Stefen Romero and Joe Dunnigan led directly to both runs in the sixth. Romero swiped his after a one-hit single, and Dunnigan drove him in with a line-drive single to left. After Dunnigan stole second, Jesus Sucre singled him home for a three-run lead.
Facing Micah Owings with one out in the seventh, Carroll went deep to left for his first Cactus League homer.
The Mariners, who split their two games in Japan in the regular-season opening series, moved to 13-8-1 in Cactus League play. The Padres, who wrap up their Arizona schedule on Sunday in Surprise against the Rangers, are 19-14.
Up next: The Padres travel to Surprise on Sunday for a 1:05 p.m. PT game against the Rangers, their final Cactus League outing of the spring. Cory Luebke, who on Friday got a contract extension from the team that guarantees him $12 million, gets the start. Luebke, who projects as the No. 2 starter in the rotation, will likely start on Friday against the Dodgers at Petco Park. He will go four innings on Sunday. Look for prospect Casey Kelly to also throw four innings. Kelly has had a strong spring and will begin the regular season with Triple-A Tucson.