Rowland-Smith (2-2) had his best career outing, going a career-high seven innings and allowing just one run on four hits. He matched his career high with six strikeouts. But the A's squeezed out the narrow victory as three A's pitchers held the Mariners to seven hits.
The Mariners, who matched their season high with their seventh straight loss, were shut out for the eighth time this season and the second time in the past three games.
"It's sort of bittersweet," said Rowland-Smith, who had 61 career relief appearances before his first career start July 1. "It's tough to walk away with a loss. But overall, I was happy with the outing."
The only run he allowed was a sixth-inning home run to Emil Brown. Brown hit a first-pitch changeup over the left-field wall for his 12th home run.
Rowland-Smith said Brown may have had a hint that a changeup was coming since he had started the first two batters in the sixth with changeups.
This is all part of the maturation process for Rowland-Smith as he aspires to be a fixture in the rotation, not just this season, but next season. He needs to vary his pitching sequences, his speed and locations and learn the mindset of a starter.
"Before after five, six innings I was sort of getting tired, not so much physically but mentally," he said. "I'm starting to get used to it. It was good to throw 110  pitches or so.
"I have to stay within myself, not get frustrated and overthrow, trying to throw 100 [mph]. Just slow it down, just stay within myself and try to execute the pitch."
"It's good to see him go deep in the ballgame and be strong at the end," manager Jim Riggleman said. "He gave us every chance to win the ballgame.
"He's been good since he has come back from Tacoma [Aug. 9]. I thought it was a good organizational decision ... to get him down there, stretched out and ready to start because he gives us a real strong option for the future."
The A's added a run in the ninth on Carlos Gonzalez one-out single, scoring Eric Patterson from third.
The Mariners had a chance in the fourth against Greg Smith (6-12) after a one-out infield single by Yuniesky Betancourt. Ichiro Suzuki followed with a single to right. But Miguel Cairo flied out to center and Raul Ibanez bounced out to third.
In the sixth, Ichiro opened with a double off the right-field wall. His three hits gave him a career .328 average against the A's, fourth best among active players. But Ichiro was left stranded as Cairo popped out on a bunt attempt. Ibanez lined out to center. After an intentional walk to Adrian Beltre, Jose Lopez hit into a fielder's choice.
The Mariners' best chance came in the ninth against Brad Ziegler. Lopez opened with an infield single. After Bryan LaHair struck out, pinch-hitter Tug Hulett doubled into the right-field corner, with Lopez holding at third. Jeff Clement walked but Betancourt hit a grounder to third, which was turned into a game-ending double play.
"That's what he does. He gets ground balls," Riggleman said of Ziegler. "It's a credit to him. He wouldn't give in. He kept sinking the ball and eventually he was going to get someone to get a ground ball and he did."
Ziegler has allowed just two runs in 44 innings since his big league promotion on May 30.
The Mariners are now 6-51 when scoring four runs or fewer. The starters are 0-12 with a 7.86 in August.
A's DH Frank Thomas was ejected for disputing the strike zone with home-umpire Bill Hohn. Thomas, unhappy after his strikeout in the second, took a first-pitch strike in his sixth-inning at-bat and said something, without turning to Hohn. Hohn immediately tossed him.