Nationals manager Dave Martinez acknowledged he was looking for someone who could end the game with one swing in the ninth inning Friday night.
He chose the right guy.
"Thought about pinch-hitting Michael (A. Taylor). He gets on, he can steal, but then what?" Martinez said. "So I said, 'Hey, we need somebody who can hit a home run right now and Mark is the perfect guy.'"
Kyle Barraclough (0-3) fell behind Reynolds in the count 3-0. Reynolds, who is 7 for his last 18 after struggling for most of June, fouled off a pitch. The Nationals slugger then sent a fastball into the visitors' bullpen in left-center for his eighth home run.
"If I'm up there and it's 0-2, I'm obviously not looking to turn on something," he said. "I knew 3-0 I was going to take a hack and then (he) pretty much threw the same pitch 3-1 and I kind of got my timing a little bit."
Four Nationals relievers combined to allow only a hit and a walk over the final four innings. Sean Doolittle (3-2) pitched the ninth for the win as Washington defeated Miami for the 13th consecutive game dating to last season.
The Nationals loaded the bases with no outs in the eighth, but failed to score.
Unlike Thursday night when Washington overcame a 9-0 deficit for a 14-12 win, big hits were harder to come by as the teams stranded 17 baserunners.
Miami starter Dan Straily lasted six innings, allowing two runs on five hits.
Gio Gonzalez threw 114 pitches over five innings and gave up eight hits and four walks. He was bailed out by three double plays and struck out Justin Bour and Garrett Cooper with the bases loaded in the fifth after walking in the tying run.
"I thought our offense did a pretty good job with Gio," Marlins manager Don Mattingly said. "We just didn't really capitalize. . Didn't get anything in the fourth and one in the fifth with chances to do more damage and didn't do anything right there. I thought that was kind of the turning point for us."
The Nationals took a 2-1 lead in the fourth on RBI singles by Adam Eaton and Wilmer Difo.