The A's roared back on RBI singles from Chris Coghlan and Matt Chapman to knot the game.
"Our guys did a really good job of battling back there and had some really good at-bats, worked the count, made them throw strikes, things we've talked about doing," said Mariners manager Scott Servais. "Unfortunately, we just didn't get it done in the last [half] inning. But it was a crazy game."
Seattle struck for two homers off A's starter Kendall Graveman, getting a two-run shot from second baseman Robinson Cano in the fourth and a solo home run off the bat of catcher Rob Brantly in the fifth, marking the end for Graveman.
The right-handed Graveman, who is expected to begin the season in Oakland's rotation, faced just one over the minimum through his first three innings before finding trouble in the fourth, finishing with three earned runs and three hits allowed across 4 1/3 innings. He walked three and struck out two.
Seattle starter James Paxton, in competition for the fifth starter spot with Nathan Karns, endured a rough audition, surrendering seven earned runs on six hits and four walks with two strikeouts in just two-plus innings to bring his spring ERA to 9.00.
After serving up Valencia's two-run shot in the first, Paxton retired each of his first two batters in the second before walking Yonder Alonso and hitting Marcus Semien with a pitch, setting up consecutive run-scoring doubles from Coco Crisp and Jed Lowrie. Josh Reddick notched a double in the sixth and went 2-for-3 to improve to 9-for-19 (.474) this spring.
New Mariners closer Steve Cishek made his first appearance since March 6 after nursing a sore biceps, firing a 1-2-3 fifth inning with one strikeout.
A's Up Next: Right-hander Jesse Hahn is scheduled to make his third start of the spring Friday in a 1:05 p.m. PT matchup with the host Indians at Goodyear Ballpark. Hahn takes a 7.20 ERA into the game, having allowed four earned runs in a combined five innings in his first two starts.
Mariners Up Next: Seattle's fifth-starter competition continues to heat up as Nathan Karns makes his fourth start of the spring in a 1:10 p.m. PT game against the Rangers at Peoria Stadium. Karns, acquired by trade from the Rays, is battling with Paxton for the final rotation berth. He's put up a 4.00 ERA in nine innings and is slated for five frames against Texas. Veteran setup man Joaquin Benoit is among the relievers scheduled in the game, which will be televised live by Root Sports and MLB.TV.