Carlos Beltran had three hits and one RBI, while the Mets scored three times after three Washington errors.
Winner Mike Pelfrey (2-0), back from a brief bout with forearm tendinitis, was touched for two earned runs and six hits in 5 2/3 innings.
He was lifted after throwing 95 pitches.
Elijah Dukes drove in both Washington runs with a fourth-inning single and a triple in the sixth, but two runs scored in the first when he committed an error after losing a fly ball in the sun.
Ryan Zimmerman and Anderson Hernandez added two hits apiece for the Nationals, who have lost three in a row.
Losing pitcher Daniel Cabrera (0-2) yielded five runs -- one earned -- and six hits in 2 1/3 innings.
Copyright 2009 by United Press International.