It was the second Washington victory in seven games.
The teams were locked in a 2-2 tie when Josh Willingham opened the Washington seventh with a single off Enerio Del Rosario (1-1).
Cincinnati center fielder Drew Stubbs then misplayed a fly ball by Roger Bernadina for an error and Desmond followed with a single that brought home Willingham. After Wil Nieves sacrificed the runners to second and third, Willie Harris delivered a sacrifice fly that brought in the second run of the inning.
Tyler Clippard (8-3) threw a shutout inning of relief to get the win and Matt Capps, despite giving up two hits in the ninth, picked up his 18th save.