Cody Stanley, called up earlier in the day from Triple-A Memphis, started the rally with a pinch-hit double. Tommy Pham walked, setting up Moss' fifth career walk-off homer and his third as a Cardinal.
Both starters went just 2u2154 innings. Cardinals lefty Marco Gonzales gave up four runs on seven hits, while Nationals righty Joe Ross gave up three runs on one hit, but he issued six walks.
Mitch Harris, also called up earlier in the day from Memphis, got the win with two scoreless innings of relief.