The estimated distance, according to ESPN's Stats & Info was 407 feet, but more than the pure distance, Stanton was glad the shot meant something.
"These are the most important," Stanton said. "The ones that go super far and are one or two runs and we lose, they are just personal stats. When you bring a team up like this and help them win, it's worth it."
The homer was the 123rd of his career, moving Stanton into sole possession of sixth place on Miami's all-time list. He had been tied with Gary Sheffield.
"We brought the water in last year," Stanton said.
Stanton drove in five runs, and he atoned for an error he committed in the second inning that led to two unearned runs.
It was Stanton's second career walk-off slam, with the first coming on May 13, 2012, against the Mets. Miami has had five all-time walk-off slams, with the last by Jeff Mathis on June 30, 2013.
Christian Yelich extended his hitting streak to 12 games with three hits.
Reed Johnson led off the ninth with a pinch-hit single off Medina. Yelich, who was sacrifice bunting, executed a perfect bunt single. Marcell Ozuna's bunt on a 3-1 pitch was fielded by Medina, who threw to third in an attempt to force Johnson. Initially, Ozuna was called out, but a challenge was issued to see if third baseman Kyle Seager had possession. After one minute and 35 seconds, the call was overturned, and the bases were full with no outs for Stanton.
Pat Shine, the team's administrative coach who reviews all close plays, immediately took charge, saw a bobble and notified the dugout to challenge.
Earlier in the day, the Marlins got a memo regarding the interpretation of the transfer rule.
"I think at the end of the day, everybody is trying to get the calls right," manager Mike Redmond said. "We got a memo today on that exact play. It worked out tonight in our favor. It ended up being a big play. It gave us the bases loaded and a chance to win the ballgame."
The decision to have Ozuna bunt opened Redmond up to criticism, because it potentially could have put the Mariners in position to pitch around Stanton. It worked out.
"I was trying to get a couple of guys in scoring position," Redmond said. "The way we've been going, if he swings, he might ground into a double play. I wanted to get guys in scoring position.
"I still take my chances with the bases loaded and one out, and our four-hole hitter at the plate, if they walk [Stanton]."
Spotty defense again hurt Miami, as two errors -- by Stanton and Derek Dietrich -- led to three unearned Seattle runs. The Marlins also hurt themselves on the bases. Yelich was out on an overly aggressive play, rounding third and heading to home on Garrett Jones' infield single with two outs in the second inning. Adeiny Hechavarria was thrown out trying to steal in the same inning, and Jarrod Saltalamacchia was picked off first in the fifth inning.
Stanton admitted that he had the error in the back of his mind, and it motivated him to try to make up for it if he had the chance.
"I was like, if I get a chance to hit, I better take care of it," Stanton said. "For what I did earlier in the game, we wouldn't have been in that situation, if I didn't make that play."
The Mariners were making every effort to avoid facing Stanton in crucial situations. They intentionally walked him twice, but with the bases full and no outs in the ninth, they had no choice but to attack the slugger.
"It's just the way it goes unfortunately," Seattle manager Lloyd McClendon said. "We really tried to stay away from him all night. He's the one guy in the lineup that can really hurt you. We had no choice there."
Saltalamacchia provided a mammoth home run in the third inning that gave Miami a 4-2 advantage. According to ESPN's Stats & Info, the drive into the upper deck in right field was estimated at 429 feet. The figure dramatically differed from the Marlins Park estimate of 473 feet.
In the fifth inning, Seattle clawed back to within one on Abraham Almonte's leadoff triple and Brad Miller's RBI groundout.
Almonte scored the tying run in the seventh, getting the inning going with a one-out double off Mike Dunn. Brad Miller reached on Dietrich's error at second base, but Dunn retired Robinson Cano on a popout to short. A.J. Ramos entered to face Corey Hart, and the reliever quickly got the count to 0-2. The next pitch, Hart poked a slider that was down past a diving Hechavarria at short, driving home the tying run.
Twice Miami had leads of two runs for Nathan Eovaldi, who gave up three runs (one earned) in six innings in a no-decision.
Eovaldi worked out of trouble in the sixth inning, after he issued a leadoff walk to Michael Saunders and a single to Seager. The right-hander induced a double-play grounder out of Dustin Ackley and struck out Mike Zunino, preserving a one-run lead.
Miami got to Chris Young early, scoring twice in the first inning. Ozuna doubled and scored on Stanton's RBI single. A wild pitch moved Stanton to second, and he scored on Casey McGehee's RBI single.
As they've done throughout their rough two weeks, the Marlins allowed a costly miscue to lead to unearned runs.
Saunders singled sharply to open the second inning, and after Seager lifted a long flyout to right center, the Mariners tied it on Ackley's single to right field. The ball skipped past Stanton and rolled to the wall. Saunders scored from first, and Ackley rounded the bases and scored standing up. The three-base error erased Seattle's early deficit.
"We haven't had a lot of breaks go for us, but part of that is our fault, too," Redmond said. "We've probably had more breaks go against us. We've been a little sloppy in areas. We had some baserunning mistakes tonight. We had some defensive mistakes tonight. At the end of the day, we were still able to get a win. Hopefully that will give us some momentum going forward."