That wasn't determined, though, until the umpires consulted replay.
It was the first walk-off homer of McCann's career and the Braves' 23rd victory in their last at-bat this season.
The ball was first ruled in play for a double, but the umpires checked the replay and ruled the home run. As McCann completed the trip around the bases, his teammates and Braves fans at Turner Field could finally officially celebrate.
"It was definitely weird," McCann said.
And a big victory for the National League East-leading Braves, who are 46-18 at home and are 16-0-2 in their past 18 series there since losing two of three games against the Phillies in April.
The Braves scored three times in the eighth inning, the last two thanks to a Marlins error, to get within striking distance. Then came the drama.
The Braves had come from seven runs down in the ninth inning to beat the Reds on a grand slam by Brooks Conrad in May, but this was almost as stunning.
Leo Nunez walked Conrad, and Diaz followed with his opposite-field homer to right field to tie the game. Two outs later and with extra innings looming, McCann won it.
Down in the count 1-2, McCann got a pitch to his liking and sent a drive to right field. Had the ball gone over the fence or not?
"I knew it was out," McCann said. "I heard it hit the tin on the back wall. It makes a different sound."
But second-base umpire Tim McClelland at first singled that the ball was in play, and McCann stopped at second base.
The umpires conferred and then left the field to view the replay. After 86 seconds, they emerged and McClelland, the crew chief, signaled home run.
"I didn't care how long it took," McCann said.
Given the homer, McCann strolled around the bases and the delayed celebration actually seemed muted. Maybe everyone was stunned.
The Marlins didn't argue.
"[The ball] made a loud sound," Marlins right fielder Mike Stanton said. "You could tell -- there is no clank in play. The fence is soft. It's not going to make a loud noise."
The Braves' rally got Derek Lowe off the hook. He gave up five runs in three innings, walking three to go along with six hits. Only 42 of his 78 pitches were strikes.
In contrast, Josh Johnson allowed just three hits and an unearned run over six innings for Florida, striking out eight. But he left after 103 pitches and the Marlins' defense and bullpen imploded.
"If he would have finished the sixth inning under 100 [pitches], yes, I would have sent him back," Marlins manager Edwin Rodriguez said. "But over 100, he was fighting for six innings. You have a five-run lead. At some point, the bullpen has to step up."
The Braves' three runs in the eighth inning were charged to Will Ohman, with the last two scoring when first baseman Gaby Sanchez dropped a throw from shortstop with two outs.
It went bad for Lowe from the start. He got ahead of Marlins leadoff hitter Emilio Bonifaco, only to walk him with three straight balls. Logan Morrison followed with a single, and then Lowe made a 3-2 pitch too good.
Hanley Ramirez connected with a changeup that drifted into his prime hitting zone and smacked his 18th homer of the season over the left-field fence. Before Lowe had got an out, the Marlins led, 3-0.
Dan Uggla doubled off the left-field fence after Ramirez's homer and Chad Tracy walked with one out. Lowe did finally get out of the 37-pitch inning by striking out Stanton and Brad Davis.
The Braves scored an unearned run off Johnson in the bottom of the first inning on an RBI hit by Eric Hinske. But Lowe gave up two runs in the third before getting an out. Tracy drove in the runs with a bases-loaded single.
Cristhian Martinez relieved Lowe to start the fourth inning and quickly gave up a run, as Donnie Murphy followed Bonifacio's double with a one-out triple. But the reliever settled down after that and finished with three scoreless innings to keep the Braves as close as they were.
"It didn't look good for a long time," Lowe said.
But comebacks, especially at home, have been the Braves' specialty.
"It doesn't get any better than this," Diaz said.
"What a way to get your first walk-off," McCann said.