John Jaso homered in the first inning and knocked in four runs, and Mike Carp also went deep in Seattle's 6-1 victory over Kansas City.
Blake Beavan (6-6) allowed four hits and a run and fanned six to win his third consecutive start.
"(Blake Beavan) had a good fastball tonight, did a good job using it, (and) did a good job of mixing in his slider as well," Mariners manager Eric Wedge said. "He pitched a good ball game, was really efficient."
The Mariners, who moved to 5-1 against the Royals this season, have captured five of their last seven overall. Seattle clinched winning the season series against Kansas City for the first time since 2007.
Jeremy Guthrie (0-2), who was traded on July 20 from Colorado, continued his struggles. The right-hander allowed 11 hits and six runs over 5 1/3 innings. He has dropped his last eight decisions and hasn't won since May 31 versus Houston. Guthrie is 3-11 overall in 2012.
Salvador Perez drove in the lone run for the Royals, who lost for the fifth time in six tries.
"We just haven't been swinging the bats lately," Royals manager Ned Yost said. "Beavan did a great job throwing strikes, not walking guys, was ahead of the count for the most part. It's hard being successful every night when you start off 4, 5 or 6 runs behind in the first 2 innings."
Michael Saunders and Jesus Montero singled with one out in the first prior to Jaso's fifth homer of the year, a blast to right for a 3-0 lead.
Perez's sacrifice fly plate Billy Butler in the second, but the Mariners scored twice in the bottom half, one on a Dustin Ackley double and another on Jaso's sac fly.
Carp led off the home sixth with his fifth round-tripper of the year, to right.
Beavan escaped a jam in the fifth. With men on first and second and two down, Chris Getz flied out to left.
Lucas Luetge worked out of trouble in the seventh by retiring Eric Hosmer on a grounder to strand a pair of runners.
Beavan is 3-0 in three lifetime games against the Royals...Randy Johnson and Dan Wilson will be inducted into the Mariners Hall of Fame in a pregame ceremony on Saturday...Seattle took three of four from the Royals in Kansas City last week...Bruce Chen pitches for KC on Saturday, with Kevin Millwood going for the Mariners in the third portion of the four-game series...Guthrie has a 7.76 ERA over his last 13 games (9 starts) since June 5.