Seager, who has reached base in nine of his last 10 plate appearances, laced a one-out double to center off Tanner Scott (0-1). That scored Mitch Haniger, who opened the extra frame as the automatic runner on second base.
Seattle won its third in a row overall. The Orioles have dropped four straight and six of seven since opening with a three-game road sweep of Boston.
Rafael Montero (1-0) earned the victory despite surrendering the lead in the seventh inning. Kendall Graveman pitched the eighth for his first career save.
Ty France and Tom Murphy both homered for Seattle.
Justus Sheffield, who was originally scheduled to face Baltimore on Monday before the game was rained out, didn’t allow a ball out of the infield in the first four innings. He yielded two runs and three hits, including Ramón Urias’ two-run homer in the fifth, while striking out five over six innings.
Montero retired the first two batters in the seventh, but Ryan Mountcastle doubled and then scored on DJ Stewart’s RBI single to extend the game. It was Montero’s third blown save in five tries.
Baltimore starter John Means allowed three runs in five innings while striking out five. France hit a one-out homer in the first and Murphy led off the second with his first home run of the season. But Means retired 10 of the last 11 batters he faced to keep the Orioles from severely taxing their bullpen.
Mariners: LHP James Paxton will undergo season-ending surgery. Paxton left his April 6 start — his first since returning to Seattle on a 1-year, $ 8.5 million deal —after 1 1/3 innings with left elbow discomfort. … Manager Scott Servais said OF Kyle Lewis (right knee) took some at bats in an alternate site game Monday and said it was “very realistic” to expect Lewis to return in the next week.
Orioles: Manager Brandon Hyde said he is optimistic OF Austin Hays (right hamstring strain) can log some game action at the team’s alternate training site in the next few days. “Hopefully that goes well, and we’ll see him sometime shortly after that,” Hyde said.
Baltimore added RHP Travis Lakins Sr. as its 27th man for the doubleheader. Lakins pitched a scoreless inning in the opener. Seattle brought up INF Donovan Walton, who hit .154 with three RBIs in five games for the Mariners last year.
Mariners: RHP Justin Dunn (0-0, 5.79 ERA), who allowed three runs in 4 2/3 innings against the Chicago White Sox in his lone start of the year, faces an AL East for the first time in his career.
Orioles: RHP Matt Harvey (0-1, 5.59) has not earned a victory since he kept Seattle to a run in 5 2/3 innings on July 13, 2019, in his only career appearance against the Mariners.