Haren (7-11) limited the Phillies to two runs and four hits over seven innings for his first career win against Philadelphia in eight starts. He struck out seven without issuing a walk.
John Lannan (3-5) allowed eight runs, nine hits and five walks over five innings. The Phillies have lost seven of their last nine games.
Adam LaRoche snapped a 0 for 16 lifetime skid against Lannan with an RBI double in the fourth. Denard Span's two-run single capped a four-run fifth inning. Bryce Harper added an RBI single.
The Nationals went 6 for 10 with runners in scoring position.
Rendon hit a two-run homer off Lannan in the second inning after the umpires confirmed it on instant replay. Three batters later, Zimmerman hit his 13th of the season, which landed several rows deep in the left-center stands.
Washington scored only six runs during a three-game sweep by the NL East-leading Braves this week.
Rather than keep Lannan as a potential fifth starter, Washington signed Haren to a lucrative one-year deal this offseason.
After struggling for the first part of the season, Haren has started to find his groove. He's 3-0 with 1.29 ERA in his last three starts.
Before Friday's matchup, Haren sported a 0-4 record with a 5.54 ERA in seven career starts against Philadelphia. That included three starts this season, all ending in Washington losses.
This time he held the Phillies without a hit until Michael Young's one-out double in the fourth. Chase Utley followed with an RBI single, cutting the lead to 4-1. Philadelphia did not score again until the seventh by which time the Nationals had plated eight.
Lannan spent his first six major league seasons with the Nationals. Known as a groundball pitcher, he allowed multiple home runs for the first time in 13 starts this season.
The Nationals went 1 for 19 with runners in scoring position against Atlanta. They started down that scuffling path in the first as LaRoche struck out with the bases loaded after Lannan issued two walks and a single.
Another scoring chance came in the second. With Wilson Ramos on second base, Rendon hit his sixth home run for a 2-0 lead.
Consecutive doubles by Ian Desmond and LaRoche made it 4-0.
LaRoche also singled off Lannan in the fifth, setting up Ramos' RBI double. Haren's bases-loaded walk came before Span's two-run single in the fifth.
Ryan Mattheus and Ian Kroll each pitched one hitless inning of relief for Washington.