Will Smith retired two batters in the eighth for the Brewers and closer Jeremy Jeffress, a Virginia native, pitched the ninth for his 22nd save as a light rain fell.
The Brewers (36-46) snapped a four-game losing streak while first-place Washington (50-34) lost for just the second time in nine games.
Guerra (6-1), who made his major league debut at the age of 30 last year, entered the game with a 3.25 ERA in 11 starts this season. He got out a jam in the sixth as he fanned Wilson Ramos for the third out with runners on second and third.
A former pitcher in Italy and Mexico, Guerra fanned seven and walked two while throwing 109 pitches. In his previous start he did not allow a run in eight innings against the Los Angeles Dodgers.
Maldonado hit a solo homer to left on a 2-1 fastball from Max Scherzer in the fifth to give the Brewers a 1-0 lead. It was just third homer in 61 at-bats this season for Maldonado, who has only 23 in a career that began in 2011.
Scherzer (9-6) gave up one run on four hits with three walks and seven strikeouts. He went six innings and threw 114 pitches before he was lifted for pinch-hitter Clint Robinson, who grounded out after his long drive to right went foul earlier in the at-bat.
Scherzer has allowed a league-high 21 homers this year and many of them are slugged by nonpower hitters. He allowed two homers at Milwaukee on June 24. One was by Keon Broxton, who has just one homer in 64 at-bats this year, and the other was by Kirk Nieuwenhuis, who had five homers in 185 at-bats.
Ryan Braun appeared to have a clean single to center with two outs in the top of the first as the third batter of the game. But Braun was ruled out after Washington manager Dusty Baker pointed out to the umpires that Braun batted out of turn.
Jonathan Lucroy was supposed to bat third for the Brewers. Braun then led off the top of the second after Scherzer was credited with three straight outs in the first since Braun batted out of order.
Scooter Gennett had two hits for the Brewers. Washington first baseman Ryan Zimmerman entered the game with a batting average of .154 in his previous 15 games. He was hitless in four trips Monday and made the last out on a grounder back to the mound.
NOTES: The Brewers' plane landed at about 11:30 p.m. ET on Sunday in Northern Virginia after they ended a series in St. Louis. The first pitch was thrown Monday less than 12 hours later. ... Washington RHP Jonathan Papelbon (right intercostal strain) came off the disabled list Monday and OF Michael A. Taylor was sent to Triple-A Syracuse. Taylor is hitting .225 in 191 at-bats with 64 strikeouts for the Nationals while Papelbon has 16 saves. ... The Brewers selected the contract of INF Will Middlebrooks from Triple-A Colorado Springs and transferred LHP Chris Capuano (left elbow soreness) to the 60-day disabled list. OF Keon Broxton was sent to Colorado Springs after Sunday's game. ... Brewers RHP Zach Davies (5-4, 4.22 ERA) will face Washington LHP Gio Gonzalez (4-7, 4.81 ERA) on Tuesday in the second game of the series. ... After hitting five home runs with a club-record 15 RBIs in the last four games, Washington SS Danny Espinosa entered Monday with 18 homers and 49 RBIs this season. ... Nationals first base coach Davey Lopes was the manager of Milwaukee from 2000-2002.