Zimmerman (2-3), who logged his first win since April 25, mostly baffled the Reds for seven innings. The right-hander scattered five hits and a walk and struck out nine, allowing just one Cincinnati runner to reach home.
Tyler Clippard tossed a scoreless eighth and Henry Rodriguez struck out the side in the ninth for his eighth save as the Nats improved to 21-12 and maintained their half-game lead over Atlanta atop the National League East.
Danny Espinoza and Wilson Ramos each cracked solo homers to account for Washington's offense.
Jose Arredondo (2-1) was saddled with the loss after allowing Ramos' tie-breaking shot in the sixth.
Brandon Phillips got the only RBI for Cincinnati, which fell for the third time in four games.