Bryce Harper doubled twice for Washington, which completed consecutive series victories for the first time since winning three straight in late June.
Jordan Zimmermann (10-8) allowed four runs over 5 2-3 innings.
Matt Garza (6-14) gave up seven runs over 4 2-3 innings in his shortest outing since May 16. He allowed eight hits and a career high-tying six walks and raised his career ERA 7.17 against Washington in eight starts.
Jonathan Lucroy, Scooter Gennett and Khris Davis homered for Milwaukee.
Zimmermann was better than Garza, but also battled, leaving in the sixth after Elian Herrera's RBI double. He allowed eight hits, struck out seven and walked one over 116 pitches.
Washington took a 5-2 lead in the third, scoring four times with two outs.
Rendon walked, Harper doubled down the third-base line away from the shift and Ryan Zimmerman walked on four pitches to load the bases.
Ian Desmond's chopped single to the mound scored a run and Espinosa cleared the bases with a double off the very top of the out-of-town scoreboard in right center.
Washington chased Garza in a four-run fifth to make it 9-3.
Ramos drove his 10th homer of the season to center off Garza, and Rendon hit his third. Rendon homered for the second straight day.
Nationals: CF Denard Span (back tightness) and 2B Dan Uggla (back spasms) were in the lineup for a third consecutive day Sunday afternoon on rehab assignments with Double-A Harrisburg. Span started at DH.
Brewers: RHP Wily Peralta (4-7, 4.48) looks for his third consecutive victory Tuesday against Cleveland as Milwaukee begins its final interleague series of the year ag. The Indians will start RHP Josh Tomlin (1-1, 2.03 ERA).
Nationals: RHP Stephen Strasburg (7-6, 4.22), who has allowed three earned runs in his last three starts, will likely get the nod Tuesday against RHP James Shields (9-5, 3.74) and San Diego.