St Louis reached the post-season after watching the Pittsburgh Pirates edge the Milwaukee Brewers 1-0 earlier in the day.
And while that win saw the Pirates slash the gap on the Cardinals to two-and-a-half games, it also booked St Louis' fourth consecutive play-off appearance.
The Cardinals, who finish the regular season with road trips to Chicago and Arizona, were undone by Jay Bruce's big hitting.
Bruce hit two of Cincinnati's four home runs – Todd Frazier and Devin Mesoraco adding the others.
St Louis are the only National League team to have qualified for four play-offs in a row.
Russell Martin's RBI single helped Pittsburgh edge closer to joining the Cardinals.
Josh Donaldson's two-run walk-off homer kept the Oakland Athletics in the lead of the wildcard standings, as they beat the Philadelphia Phillies 8-6.
New York Yankees pitcher Masahiro Tanaka returned from more than two months on the sidelines with an elbow injury in his side's 5-2 win over the Toronto Blue Jays.
Tanaka conceded just one run in more than five innings in a match that saw Brett Gardner hit the Yankees' 15,000th homer in franchise history.
Cross-town rivals the New York Mets were the biggest winners of the day, thrashing the Atlanta Braves 10-2, with Curtis Granderson scoring three runs for the Mets.
The Chicago White Sox also scored 10 in their five-run win over the Tampa Bay Rays.
The Kansas City Royals bounced back from successive losses against the Detroit Tigers to win 5-2 with Omar Infante scoring twice.
Elsewhere, the San Diego Padres beat the San Francisco Giants 8-2 and the Los Angeles Dodgers were too good for the Chicago Cubs, winning 8-5.
And there were 8-3 wins for the Colorado Rockies and the Houston Astros, who beat the Arizona Diamondbacks and the Seattle Mariners respectively.
The Cleveland Indians beat the Minnesota Twins 7-2 and the Texas Rangers won a thriller, edging the Los Angeles Angels 2-1.
Washington also won 2-1, the Nationals defeating the Miami Marlins, while the Boston Red Sox outlasted the Baltimore Orioles 3-2.