Wang, a two-time 19-game winner with the New York Yankees, has won just two of nine starts with the Nationals since returning from a variety of arm injuries in the last few seasons.
He was out of the majors entirely in 2010 and made it back with a July 29 start against the New York Mets, dropping an 8-5 decision after giving up six runs in four innings.
Wang won consecutive outings against the Chicago Cubs and Cincinnati to get to 2-2 on the comeback on Aug. 9 and 16, but is 0-1 in five starts since, allowing 16 runs on 34 hits in 28 1/3 innings.
He won his lone career meeting with the Marlins after allowing four runs in seven innings with two strikeouts and no walks.
Washington's Stephen Strasburg tossed six solid innings, allowing one run on four hits, while fanning three. He threw 61 pitches, 45 for strikes, in a no- decision.
Clay Hensley (5-6) came on in the 12th and struck out the side to pick up the victory.
Collin Balester (1-4) pitched well in two frames of work in extra frames, but struggled in his third inning of duty. He allowed three Marlins to cross the plate in the 13th to take the loss.
Florida starter Chris Volstad had one of his better outings of the season. He threw seven innings, surrendering six hits, while walking two. He struck out three.
The Marlins start 21-year-old rookie lefty Brad Hand, who hasn't won in six starts since July 7.
The second-round draft pick from 2008 was 0-3 with a no-decision in his initial four games in the majors before a masterpiece against Houston, in which he allowed two hits and struck out four over seven innings of a 5-0 win.
But he's not recaptured the magic since, instead going 0-4 in his last four assignments.
Hand pitched 3 2/3 innings against the Nationals on July 28 and gave up a run on three hits in a game Florida ultimately won, 5-2.
The Marlins are 4-7 in his 11 starts.
Florida is 9-4 versus the Nats this season.