The estimated distance of the long ball was 442 feet.
"I don't know where that came from," Ross said.
But the catcher knew it meant a slingshot-like two-game swing. Cincinnati went from inches from being 4 1/2 games out to 2 1/2 games behind the first-place Cardinals in the tight National League Central race.
The Reds now have a chance to win the four-game series with another victory over St. Louis on Thursday afternoon.
"Games like these are character builders," said infielder Rich Aurilia, who worked a four-pitch walk from Isringhausen to set up Ross' game-winner. "Hopefully, we can get a little momentum here, a little roll and keep playing good baseball."
As his home run ball sailed into the night, Ross raised his arms above his head as he reached first base and the sellout crowd of 41,649 at Great American Ball Park roared. After he rounded third base, the Reds catcher launched his helmet sky high before he was mobbed at home plate by teammates.
"I saw it go out and I couldn't even hear myself round the bases," Ross said. "It's a lot of fun rounding third and seeing all my teammates waiting on me. It was definitely a rush."
Many in the home plate welcome party played a hand in getting the Reds in position to win a game they seemed destined to lose early on.
Starter Aaron Harang gave up Jim Edmonds' first-inning grand slam and fell into a 4-0 hole against Cardinals ace Chris Carpenter. But Harang and six Cincinnati relievers kept the game from getting out of reach while the lineup chipped away.
"Any time you get down four runs to Chris Carpenter and you're able to come out with a win, it's a big victory," Reds manager Jerry Narron said. "You're talking about one of the best pitchers in the game."
"We've got down early and all year, if we can sniff it, we'll make it close in the end," said Ross, who entered the game as a pinch-hitter in the seventh inning.
Carpenter, who gave up six earned runs over seven innings, allowed three solo homers. Adam Dunn hit a long ball to left field in the second inning. Scott Hatteberg and Edwin Encarnacion cleared the fences in the fourth and made it a one-run game.
Harang gave up Scott Spiezio's two-run homer in the fifth, but came up with some big outs in the sixth. After he issued a two-out walk, the Reds' ace struck out Albert Pujols and ended the inning without a run scoring.
"Our best pitcher against the great Pujols, the best player in the game," Narron said. "Shoot, it was a big confrontation. Harang got him out."
"I think it got the guys pumped up," said Harang, who gave up six runs, five earned, in six innings. "I got us in that situation, but I'm still bearing down. In that situation, you can easily want to hand the ball over or give in and not focus or execute your pitch. I knew I had to get him out in that situation."
Brandon Phillips' sixth-inning two-run double tied the game and erased Carpenter's chance for a win. In the eighth, Reds reliever Bill Bray gave up David Eckstein's RBI single that put St. Louis up by one run.
In another bases-loaded jam later in the same inning, Kent Mercker was brought in and got his only batter -- Edmonds -- to fly out and end the threat.
Narron nearly exhausted his bench using pinch-hitters, and almost emptied his bullpen. Coming off working 4 1/3 innings of long relief in Monday's blowout loss, Ryan Franklin (2-5), pitched a 1-2-3 ninth inning for Cincinnati.
"It was huge," Narron said.
Isringhausen (3-5), who entered in the eighth to attempt to get the final five outs for St. Louis, faltered with four straight balls to Aurilia. The rest was up to Ross, who did not miss and gave the Cardinals closer his eighth blown save in 37 chances.
Ross said he previously hit one other walk-off homer that helped the Dodgers clinch a tie for the playoffs.
"It was another big one in my career. It didn't feel quite like this one for some reason," said Ross, who has 14 homers this season. "Maybe [the fact that] I sat on the bench for seven innings had something to do with it. I was eating seeds, chewing bubble gum and Rice Krispie treats or something. It was definitely exciting. My voice is hoarse from screaming so much."