Seager's homer and Justin Smoak's solo shot in the ninth were the only extra- base hits in a game mostly controlled by the starting pitchers.
Seattle's Hisashi Iwakuma (11-6) gave up one run on four hits while walking three and striking out eight in seven innings of work, while Derek Holland was denied a win despite allowing just two hits and three walks in seven scoreless innings for Texas.
"I did a good job mixing speeds and keeping guys off balance tonight," Holland said. "Unfortunately we couldn't come out on top."
The Mariners improved to 2-2 on their nine-game road trip and handed the Rangers just their third loss in their last 17 games.
A.J. Pierzynski had driven in the contest's only run when he knocked in Ian Kinsler with a base hit to left field in the fourth inning.
Neal Cotts (4-2) couldn't hold the Rangers' slim lead, however. The left- handed reliever walked Nick Franklin with one out in the eighth, then caught too much of the plate on an 0-2 fastball to Seager, who lined the mistake just over the wall in right center for his 18th home run of the season.
"I think (Cotts) caught a little more of the plate than he wanted to," said Seager, who had been in a career-long 0-for-21 slump prior to the blast.
Yoervis Medina entered with two on and no one out in the bottom half after Oliver Perez walked and hit the first two batters of the frame. After a sacrifice bunt, Medina struck out Kinsler on a payoff pitch and intentionally walked Adrian Beltre to load the bases.
Charlie Furbush left the bags full by retiring Pierzynski on a pop out to third, and Smoak took Tanner Scheppers deep to right for an insurance run in the away ninth.
Danny Farquhar retired the side in order in the home turn to post his sixth save.
The Rangers nearly pushed a run across in the first inning, but Elvis Andrus was thrown out trying to score from first on a routine single to right. Franklin made the replay throw and Humberto Quintero applied a swipe tag that both Andrus and Rangers manager Ron Washington argued was never applied.
Smoak and Beltre each had two of their respective club's four hits ... With two home runs, the Mariners tied the Blue Jays for the second most home runs in the majors (146) ... The Rangers still lead the season series, 8-6.
08/17 01:35:08 ET