It gave the Dodgers a 6-5 winning margin and a sweep of the defending World Series champions, who have now lost seven straight to Los Angeles.
"That was cool," Van Slyke said of Mattingly's decision to give him the green light on a three-ball count. "I don't think he was going to pinch-hit me to walk or get a little single. I think he wanted me to do some damage."
And Van Slyke did just that. Heading into his at-bat, the Dodgers had put runners on base throughout the night with 13 hits. But 12 of those were singles, leading to missed opportunities and leaving the Dodgers trailing by two.
That's when Mattingly elected to go with Van Slyke to avoid a lefty-lefty matchup between Adam Kennedy and Cardinals reliever Mark Rzepczynski. And, as Van Slyke said, to find some power.
"That's what he's up there for, really," Mattingly said. "He can hit a ball in the gap, and it seemed like the right thing to do. I sent him up there to hit."
The 25-year-old Van Slyke is the son of former big leaguer Andy Van Slyke, who was in the stands to watch his son's pivotal homer. Andy Van Slyke played four seasons with the Cardinals in the '80s before winning five Gold Gloves and making three All-Star Games with the Pirates.
"Not only hitting the home run, but having it mean something and helping the team win is a little more exciting than the actual home run," Van Slyke said.
The powerful rookie, who hit .336 with Triple-A Albuquerque before being called up on May 9, said he approached the at-bat simply trying to hit a ball into the gap. But when the count went to 3-0, he changed his mindset to "give myself one shot to take the lead."
Prior to Van Slyke's heroics, Dodgers starter Chad Billingsley allowed five runs (three earned) in six innings, an improvement from his two previous starts, in which he lasted four innings in each.
He seemed to settle down after a rocky first two frames -- including a second inning in which he worked out of a bases-loaded, no-out jam with a strikeout and a double play. But later in the game, he was done in by a series of bloop hits and a couple of errors, one of his own volition.
"I felt like I had pretty good command of throwing the ball where I wanted it to go," he said. "I had a good fastball and good offspeed stuff. It's just the way the game is, you can't do anything about it."
Billingsley left the game with the Dodgers trailing, 5-2, but they fought back, first with James Loney's RBI single to score Kennedy in the sixth, then the home run an inning later. Bobby Abreu and rookie second baseman Elian Herrera each had three hits.
But the biggest hit by a Dodgers rookie on Sunday was Van Slyke's.
"He's coming up there to get a pitch to hit," Rzepczynski said. "I threw a 3-0 changeup thinking he would be a little bit out in front, and he sat back and hit it really well."
Kenley Jansen, coming off a blown save on Friday, put down the Cardinals 1-2-3 in the ninth inning for his fifth save. The victory improved the Dodgers to 28-13, the best record in baseball with more than a quarter of the season gone.
The value of a sweep of the Cardinals was not lost on Mattingly, who isn't one to typically assign one regular-season game more value than another.
"We're banged up, and obviously that's a good team over there," Mattingly said. "They've got a lot of talent. They've had some injuries, but they're pretty deep down through that order. It's a good feeling to be able to beat these guys."
But even with his roots in Chesterfield, Mo., Van Slyke downplayed the meaning of hitting a game-winning home run against the Cardinals.
"I have a couple of friends in the stands and we're all from St. Louis, so I'm sure a little piece of them was cringing," Van Slyke said. "But I think whatever team it was, it would have been just as thrilling."
Just as thrilling, maybe. Just as "storybook," almost certainly not.