The Mariners overcame a shaky start from ace Felix Hernandez to win the opener 6-4 on Justin Smoak's two-out single in the 10th inning.
Yoenis Cespedes homered for the second straight day and Craig Gentry scored from first on a single by Jed Lowrie to help the A's snap a six-game home losing streak.
Pomeranz (2-1) was sharp in his first major league start since last July, allowing two hits and no walks in five innings with five strikeouts. Pomeranz won just one of his final 19 starts with Colorado, leading to an offseason trade to the A's where he has pitched well in relief. But he stepped when the A's needed another starter for the doubleheader and got the win.
The A's took the lead in the third. Gentry ran on the pitch from first base on a full count with two outs when Lowrie's popup fell between left fielder Cole Gillespie and shortstop Brad Miller for a hit. Gentry never stopped running and scored easily as Gillespie appeared screened by Miller and fielded the ball with little urgency.
Cespedes added to the lead in the fourth with a solo homer off Erasmo Ramirez (1-4), who was called up from Triple-A Tacoma earlier in the day to make the start. Ramirez allowed two runs and six hits in six innings.
Dan Otero, who allowed the go-ahead hit in the 10th inning of the opener, pitched three scoreless innings of relief and Jim Johnson finished the three-hitter for his second save and first since losing the closer's job a month ago.
This was just the second traditional doubleheader in Oakland in the past 16 years, with the other in 2011. The makeup game was necessary because a game was postponed April 4 when the field was unplayable after there was no tarp on it overnight.
Hernandez failed to strike out a batter for the first time Aug. 19, 2008, against the Chicago White Sox, ending a streak of 179 straight starts with at least one strikeout. He allowed four runs three earned and matched his career worst against Oakland by giving up 11 hits in 6 1-3 innings.
"No strikeouts, not a good breaking ball, not a good change-up ," Hernandez said. "That's what happens when you don't have good stuff."
Brandon Moss hit an RBI triple that capped a three-run seventh against Hernandez, but the A's could not hold a late lead as Robinson Cano tied the game with an RBI single off Luke Gregerson in the eighth.
Smoak gave the Mariners the lead when he hit a chopper off the glove of leaping first baseman Daric Barton for an RBI single off Otero in the 10th after Ryan Cook (0-1) left the game with a strained right forearm.
"I was pretty mad about it as soon as I hit it," Smoak said. "Once it clanked off his glove I was just trying to get to first."
Yoervis Medina (2-1) stranded two runners in the ninth for the win, and Fernando Rodney pitched the 10th for his ninth save in 10 chances.
Oakland center fielder and leadoff hitter Coco Crisp left in the fourth inning of the opener with a strained neck. Crisp got hurt after crashing into the wall to make a leaping catch of Seager's two-out drive. He is day to day.
NOTES: Corey Hart and Mike Zunino hit solo homers for the Mariners in the opener. ... Hisashi Iwakuma (1-0) takes the mound for the Mariners on Thursday when they open a four-game home series against Kansas City. ... Oakland begins a three-game series at home against Washington on Friday.