The Cards had scored four times to take the lead in the bottom of the eighth inning, before the Cardinals responded with a pair of runs in the top of the ninth to tie the game. That would set up the game-winner for DeJong, as Koda Glover suffered his first loss after giving up the blast.
Ryan Madson gave up a walk-off grand slam on Sunday night against David Bote of the Chicago Cubs. The Nats' bullpen has now blown saves in three of the past four games, as their closer Sean Doolittle has been on the disabled list since July. Kelvin Herrera, who was acquired from the Kansas City Royals, landed on the DL last week.
The Cardinals recorded their 10th walk-off victory of the season, as DeJong became the sixth different player to walk things off for them this year. Matt Carpenter and Jedd Gyorko both went deep in the eighth inning to power the first Cards' rally. Closer Bud Norris surrendered the lead on RBI singles from Daniel Murphy and Matt Wieters, as Dakota Hudson earned his third win after working out of that jam.
The Cardinals are now nine games over .500 after winning six in a row, as their record is this good for the first time this season. That has them just five games out of first place in the NL Central, and just two games out of the final National League Wild Card spot. The Nats got home runs from Bryce Harper and Juan Soto in the loss and are six games out in the Wild Card race, as they will continue their series in St. Louis on Tuesday night.