The Nationals beat the Los Angeles Dodgers 4-1 to bring post-season baseball to Washington for the first time since 1933.
The victory secured a National League wild card on the back of Ross Detwiler's six strong innings and Ryan Zimmerman's RBI double.
Washington reduced their magic number to win the NL East to eight and extended their advantage over Atlanta to five and a half games.
But the Nationals were not the first team to clinch a play-off spot after that had been achieved earlier in the day by the Cincinnati Reds.
The Reds beat the Chicago Cubs 5-3 with manager Dusty Baker absent and in hospital after being diagnosed with an irregular heartbeat.
Cincinnati, leaders of the NL Central, ensured at last a wild card spot and moved to within two wins of taking the title for the second time in three years.
Johnny Cueto (18-9) pitched six shutout innings as the Reds completed a three-game sweep. Alfredo Simon got four outs for his first save.
The Reds broke a scoreless tie by getting five straight singles off Manuel Corpas in the seventh during a five-run rally capped by Henry Rodriguez's two-run double.
The St Louis Cardinals beat the Houston Astros 5-4 for a 6-0 sweep at home against their opponents as pinch-hitter Carlos Beltran snapped a sixth-inning tie with a two-run double and Allen Craig hit a three-run homer.
The Milwaukee Brewers rallied for a 9-7 win over the Pittsburgh Pirates with Rickie Weeks hitting a tying two-run triple and scoring the go-ahead run on Aramis Ramirez's single to make it five straight victories.
The San Francisco Giants moved to a magic number of four as the West leaders beat the Colorado Rockies 9-2 as Pablo Sandoval homered from both sides of the plate.
The San Diego Padres were 6-5 winners over the Arizona Diamondbacks with Jesus Guzman homering as Clayton Richard allowed four runs and eight hits in six innings to raise his career record against Arizona to 6-0.
Philadelphia stormed to a 16-1 thumping of the New York Mets.
The New York Yankees opened up a one-game lead in the American League East with a 10-7 win over the Toronto Blue Jays.
Ichiro Suzuki hit a go-ahead, two-run double in a seven-run fourth inning capped by Nick Swisher's grand slam.
The Yankees completed a three-game sweep of the Blue Jays to make it five wins in a row.
Toronto trailed 10-4 before a three-run eighth. David Robertson got three outs for his second save this season.
Central leaders Chicago White Sox suffered a blow to their hopes with a 4-3 defeat by the Kansas City Chiefs.
Tampa Bay beat Boston 7-4, Oakland were 12-4 winners over Detroit, Cleveland needed 10 innings to edge Minnesota 4-3 and Texas beat the Los Angeles Angels 3-1.