David Murphy hit a two-run single during a 3, game series.
Texas starter Alexi Ogando pitched two perfect innings before exiting the game. The Rangers will likely use Ogando in the bullpen for the playoffs.
Scott Feldman (2-1) gave up three runs on five hits over four innings of relief to pick up the win.
"I want to get one thing straight," Rangers manager Ron Washington said. "When I put nine guys out there that's on my roster, we can win with them, so it's not like we're trying not to win because we put this guy out to play, that guy out to play."
Felix Hernandez (14-14) allowed seven runs -- five earned -- on 12 hits over 3 1/3 innings. He left the game after Michael Young's fourth-inning line drive struck him in the right forearm.
The Rangers got on the board in the second inning. Leonys Martin hit his first career double and scored on Blanco's two-out single to left. Blanco was thrown out at second to end the frame.
Seattle plated two runs in the fourth to take the lead. Kyle Seager singled and Dustin Ackley doubled to begin the inning. Carp's sacrifice fly and Smoak's RBI single made it 2-1.
Texas went back in front for good with six runs in the home fourth. Martin singled, Craig Gentry was hit by a pitch and Blanco reached base on an infield single to load the bases. German, Chavez and Murphy hit three straight singles to plate four runs. Young's line drive off Hernandez loaded the bases again. Jeff Gray came in from the bullpen. Mitch Moreland reached base on Gray's throwing error, which allowed two more runs to score for a 7-2 game.
Alex Liddi scored on a wild pitch in the fifth to cut the gap to 7-3.
Yoshinori Tateyama, Darren O'Day and Michael Kirkman each tossed a scoreless inning of relief to seal Texas' victory.
"That was one of our worst games, if not the worst game, of the year," Mariners manager Eric Wedge said. "Unacceptable. Ugly. It's not what I'm about. It's not what we're about.
"I don't [care] if we've got three games left or 103, that's unacceptable, and we're not going to allow that to happen."
Texas is 14-4 against Seattle this season...Hernandez fell to 0-4 against the Rangers this year and 10-16 lifetime. He was diagnosed with a bruised forearm, with no fracture...Texas has won 10 of its last 12 games.