Rookie Grant Green hit a three-run double to help Jerome Williams win his fourth straight start.
The Angels won for the 21st time in 28 games. Earlier in the day, they were officially eliminated from postseason contention.
After starting the year with big expectations, boosted by the winter signing of former MVP Josh Hamilton, the injury-plagued Angels didn't spend one day atop the AL West after winning their season opener at Cincinnati.
The Angels have been no fewer than nine games out of first place since the All-Star break. This is their fourth straight year without a postseason berth, after winning five division titles.
Cowgill walked and scored in the second inning and hit a solo homer in the fourth.
Cowgill tripled to open the seventh. He then broke for the plate as Andrew Romine squared around to try a suicide-squeeze bunt, and scored when Saunders' pitch sailed to the backstop.
Williams (9-10) allowed one run, five hits and five walks in 5 2-3 innings. He left after No. 9 hitter Henry Blanco snapped an 0-for-32 drought with a bloop single.
Ernesto Frieri got three outs for his 36th save in 40 attempts despite giving up Raul Ibanez's 300th career homer and 29th this season.
Joe Saunders (11-16) tied the Astros' Lucas Harrell and the Cubs' Edwin Jackson for the most losses in the majors.
Saunders gave up six runs and six hits over seven innings and tied a career best with nine strikeouts. The 32-year-old left-hander threw 125 pitches, one short of his career high.
Abraham Almonte scored Seattle's second run on a seventh-inning balk by rookie Cory Rasmus.
NOTES: Last year, the Halos' postseason hopes ended with two games left on the schedule. ... LHP C.J. Wilson, who goes into Sunday's start with a 17-6 record and a string of nine straight winning decisions, was presented with the fifth annual Nick Adenhart memorial award as the Angels' top pitcher of the season during a pregame ceremony. Jered Weaver won it the first four years. ... Justin Smoak was hitless in three at-bats against Williams, and is 1 for 19 lifetime against him with seven strikeouts. ... The Mariners are 0-68 this season when trailing after eight innings, and 6-221 during Eric Wedge's three years as manager.