Granderson hit his third homer of the spring on the third pitch he saw from reliever Drew Storen, driving it over the fence in right field.
''I'm honestly not too sure,'' Granderson said when asked what the pitch was. ''I was just trying to get down and get a ball to drive. No ridiculous thought process or anything like that.
''He threw me three pitches, and honestly, all three were different.''
Giancarlo Alvarado earned the win in relief and Storen took the loss. Bobby Parnell earned the save.
Washington's Tanner Roark allowed a run in 5 2-3 innings with five strikeouts.
Roark is competing with fellow right-handers Taylor Jordan and former Met Chris Young after Ross Detwiler was recently moved to the bullpen. Roark dazzled the coaching staff last Tuesday in Viera with a dominating performance in a minor-league game against a group of Detroit prospects.
''I felt like I could throw any pitch in any count. I've gone out there and competed and gotten outs. I feel like I've done pretty well,'' Roark said.
The competition hinges on what develops with starter Doug Fister, who was shut down for 20 days after elbow inflammation surfaced following his Grapefruit League debut March 2 against Miami.
Fister, obtained from Detroit in the offseason, slots as the team's No. 4 starter. He returned over the weekend with 3 2-3 innings of scoreless work against the Marlins on Saturday.
''It depends a little bit on Doug. We need to see how he reacts, but we'll make those decisions after seeing how he feels and where we're at,'' said Williams, who added that Fister said Sunday morning he felt fine following his start.
''The anticipation is that (Roark, Jordan and Young) have thrown their spring training and we'll make a decision. We just don't know.''
STARTING TIME: Mets: RHP Jenrry Mejia, a longshot candidate for the rotation's fifth spot, matched Roark's outing. He gave up a run and two hits in five innings while striking out six, but came out after five innings with a bunion problem on his big toe.
''I've always had that, but I forget to (pad it) today,'' Mejia said.
Nationals: Infielder Anthony Rendon (chest cold) is being treated with antibiotics and is working out with the team. He stayed back at the club's Viera training complex but is expected back in time for opening day.
Mets: LHP Jon Niese (elbow) may be placed on the disabled list but declared himself ''pain free.'' He threw to bases from the mound Sunday and will toss a 40-pitch bullpen Monday, with a break after the first 20 pitches.
''Nobody wants to start on the DL. The only thing that matters is getting my arm right. (Manager) Terry (Collins) has preached that a few days is better than a few months. I think we've taken the right steps and approach for the issues I've had,'' Niese said.
If all goes as planned, Niese will make his regular-season debut April 6 at home against Cincinnati.
Roark received some help from left fielder Bryce Harper in the second inning when Chris Young lofted a high fly down the left field line that appeared to be headed out for a solo homer but stayed in the park.
Harper jumped for the catch but couldn't make it, then recovered to throw out Young on his way to third as the Mets center fielder was trying to stretch the hit into a triple.
Mets second baseman Daniel Murphy, in his first game back in a week after a calf injury, fielded Harper's bunt in the sixth inning and flipped it between his legs to nab the speedy Harper.