Jose Marmolejos had two hits and drove in a run, helping Seattle take two of three in the weekend series. Phillip Ervin scored twice and drove in a run.
Casey Sadler (1-0) got the win with two scoreless innings in relief of Justin Dunn, who struggled with control issues. Seattle used six relievers the day before a doubleheader at home.
''We tried to give our team every opportunity to win the ballgame,'' Mariners manager Scott Servais said. ''It happens. It's a crazy season. Certainly a day heading into a doubleheader you would like to get a little bit more length out of our starter. It didn't happen, we've got to adjust on the fly and that's what we'll do.''
Josh VanMeter had two hits for Arizona, including an RBI double. Luke Weaver (1-7) allowed two runs and four hits in five innings.
Walton put Seattle ahead to stay with a run-scoring groundout in the second. He had a big hit against Travis Bergen with two out in the sixth, hitting a two-run double to give the Mariners a 4-1 lead.
''You try to do anything you can to help the team win,'' Walton said. ''All I was trying to do was just keep it short. Just get a pitch to drive for the most part.''
Arizona got two back in the seventh on Christian Walker's forceout and Kole Calhoun's sacrifice fly. But Ervin hit an RBI double in Seattle's two-run eighth and Kyle Lewis added a sacrifice fly in the ninth.
Last-place Arizona lost for the 20th time in its last 24 games.
''Overall, the entire day, we weren't winning that first-pitch battle and that was a big difference in the line score,'' Diamondbacks manager Torey Lovullo said. ''They scored several runs with leadoff walks, and that's the name of the game.''
The Mariners jumped in front on Marmolejos' two-out RBI double in the first, driving in Kyle Seager.
The Diamondbacks tied it at 1 with Josh Rojas' hustle around the bases. Rojas slid home just ahead of the tag on VanMeter's double to right-center in the first.
Dunn walked five in two innings. He allowed one hit and struck out five.
IN THE HUNT
The Mariners are in the mix for second in the AL West and a spot in the postseason. They are 1 1/2 games behind Houston. Oakland is on top of the division.
''We continue to get better, working hard. We've got some interesting series coming up, some teams that have got winning records,'' Servais said. ''But our guys are looking forward to it. They're not afraid of anything.''
LOSING SEASON CLINCHED
The Diamondbacks, with a 17-31 record, assured themselves of a losing season in 2020 with 12 games left. It's the first one under general manager Mike Hazen and Lovullo, who both arrived before the 2017 season.
''It means we've got to be better,'' Lovullo said. ''It means I'm going to need to go back and evaluate a lot of the things that happened, a lot of the things I did. I'm not going to use 60 games as an excuse. We knew it. We knew the ground rules. It may have been different, but we had plenty of time to get used to it. It's humbling but it's going to make us better.''
AIR QUALITY CHECK
Servais said the club is monitoring the air quality in Seattle, which has been unhealthy due to smoke from wildfires throughout the Pacific Northwest. It could affect whether the Mariners play a scheduled doubleheader at home Monday.
''Sounds like the weather may get a little bit better, Servais said. ''We just have to wait and see what brings (Monday) at the ballpark.''
Diamondbacks: RHP Zac Gallen, who worked five innings Saturday, has been dealing with a fingernail problem on his right middle finger, Lovullo said. Gallen has been Arizona's most consistent starter this season. Lovullo said the effect the fingernail had on Gallen's pitching was ''minimal.''
Mariners: Head home for a makeup doubleheader with the Athletics. A three-game series at home with Oakland was postponed earlier this month due to a positive test for COVID-19 among the A's traveling party. LHP Marco Gonzales (5-2, 3.02 ERA) starts the opener for the Mariners, with the second game to be a collection of relievers with an opener.
Diamondbacks: After Monday's off day, LHP Madison Bumgarner (0-4, 7.52 ERA) is scheduled to start for Arizona in Anaheim on Tuesday against the Los Angeles Angels.
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