Willingham drove in all five runs for the Nats, while Hernandez pitched seven scoreless innings for the victory.
The first four runs Willingham drove in were scored on a three-run triple that became a grand slam off left-hander Johan Santana.
With the bases loaded and one out in the first inning, Willingham drove a 2-2 changeup off the center-field wall that bounced back onto the field of play. Two runs scored, before Adam Dunn barreled into catcher Rod Barajas, scoring a third run and knocking the ball loose.
"It looked pretty scary. I wouldn't want to have a 300-pound man running into me," Willingham said.
With the ball on the ground, Willingham also attempted to score, but Barajas managed to recover and tag him out.
Nationals manager Jim Riggleman came out and argued that the ball cleared the fence in center. After the umpires reviewed the play on video, it was ruled a grand slam, giving the Nationals a 4-0 lead. The ball clearly went over the orange line in center field.
"I really didn't know if it was a home run," Willingham said. "I was running around first when the ball hit off the wall. I really didn't see it hit off the wall. I was watching Adam in front of me and he wasn't running real fast. He thought it was a home run. There were a lot of things that happened in that play. I tried to score and got called out. For the umpires to say it was a home run, it was nice."
Two innings later, Willingham stroked a Santana fastball and drove in Cristian Guzman with a double to center field. It marked the second day in a row that one player knocked in all the team's runs. On Saturday, Willy Taveras drove in all of Washington's runs in a 4-3 victory.
After six games, Willingham is 8-for-19 [.471] with a home run and a team-leading seven RBIs. Riggleman is hoping that Willingham's success at the plate can rub off on the rest of the lineup. As a team, the Nationals are hitting .226.
"To this point, we won a couple of ballgames and we haven't got started offensively," Riggleman said. "It's good to see it happen today. Adam had some good at-bats, and he didn't have a lot to show for it. He is seeing a lot of pitches. He got the pitch count up on Santana -- made him throw a lot."
Hernandez limited the Mets to five hits and allowed only two runners to reach scoring position. He threw just 88 pitches, but acknowledged that he didn't have anything left after the seventh inning.
"He works off your aggressiveness," Mets third baseman David Wright said. "He's working really the outer half of the plate -- throwing the ball where he wanted to. And a guy like that shows you that you don't have to be overpowering to be successful at this level."
It's amazing that Hernandez, who didn't have much of a Spring Training, went as far as he did in the game. He pitched in only two Grapefruit League games, because he was trying to get into pitching shape after signing a Minor League contract with the club in late February. Hernandez didn't go more than five innings in a Spring Training game.
When the regular season started, Hernandez was left behind in order for the team to carry eight relievers. Washington didn't need a fifth starter until Sunday, because of the off-day Tuesday.
Hernandez spent most of his down time in Miami, where he lives, but had bullpen sessions and pitched in an intrasquad game in the team's complex in Viera, Fla.
"Livo said he had had enough," Riggleman said after Sunday's game. "I wasn't thinking about taking him out, but I did know in Spring Training, he never went seven innings. But it was nice day to pitch, in terms of the weather. It wasn't a draining-type day. When he came off after the seventh, he said he was done."
Hernandez said he didn't feel any added satisfaction by beating the Mets, who released him in August.
"I'm not that kind of guy. What happened last year was business," Hernandez said. "I'm not the owner of the team. They threw me off the club. I can't do anything about it. I wasn't happy about it, but you see it every day."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.