Trailing 3-1 in the seventh, the Marlins rallied for five runs off reliever Brian Broderick.
Coughlan finished with two hits, was hit by a pitch, and scored twice.
Marlins starter Adalberto Mendez pitched 3 2-3 innings, allowing one run and two hits with four walks.
Washington's Ross Detwiler gave up one run on five hits in five innings. He struck out two.
Michael Morse hit a solo homer for the Nationals, his ninth of the spring.