Milone (12-10) allowed eight hits and two runs, matched a career high with 10 strikeouts and had no walks.
He has walked one or no batters in each of his past 14 starts. That ties the longest streak by a starter in Oakland history. Gil Heredia also had 14 starts with one walk or less from May 29 to Aug. 11, 1999.
During his stretch, Milone has issued just eight walks and struck out 78 in 88 innings.
Grant Balfour worked the ninth to pick up his 16th save in 18 opportunities.
The victory moved the A's within 3 1/2 games of Texas in the AL West. The Rangers lost to Tampa Bay 6-0. Oakland is also battling the New York Yankees and Baltimore atop the wild-card race.
Jason Vargas (14-10) allowed just four hits and three runs in seven innings for Seattle. He walked three and struck out six. His critical mistake was a 1-1 pitch to Gomes in the fifth. Adam Rosales and Coco Crisp both had singled with two outs. Gomes then turned on the inside fastball and hit it against the left-field facade for his 16th home run and a 3-0 lead.
Vargas has yielded 32 home runs this season, including six in his last three starts.
Josh Donaldson hit also hit a solo home run in the ninth off Josh Kinney, his seventh.
The Mariners scored a run in the bottom of the third. Alex Liddi singled and stole second. He came home on Michael Saunders' triple into the right-field corner.
Kyle Seager made it 3-2 in the sixth with a leadoff home run, his 17th. It came on off a 1-1 curveball from Milone, who had a career-low three-inning outing in his previous start Monday against the Angels.
Notes: A's starter Brett Anderson is 14 months removed from Tommy John surgery but there is no talk yet of a possible innings restriction next season, much like Washington did with Stephen Strasburg. "It could be something we discuss but it's not on the radar yet," A's manager Bob Melvin said. "Maybe the fact that he is pitching for a month and a half this year will have something to do with next year." ... The A's said there is nothing new to report on the condition of RHP Brandon McCarthy, still in a Bay Area hospital after being hit in the head by a line drive last Wednesday. ... Felix Hernandez (4.2 IP) and Hisashi Iwakuma (3.2 IP) failed to go at least five innings in back-to-back games in the first two games of this series. It's the first time since June 19-20 that a Mariners starter did not last at least five innings in consecutive games.