It wasn't a vintage Johnson performance, as the Big Unit left after 78 pitches in six innings, while tallying just two strikeouts. But he was devastatingly effective on the mound, surrendering just two hits and one unearned run.
"To me, wins have always outweighed strikeouts because strikeouts are kind of just a thing that happen," said Johnson, who is second on the all-time strikeout list. "I wanted to be more known for winning ballgames than striking people out. Pitching a game like I did today ... I get more gratification out of that.
With a crowd on hand to watch history in the making, there was certainly some nailbiting in the bottom of the eighth.
The Nationals loaded the bases with two outs for cleanup hitter Adam Dunn, who ran the count full against Giants closer Brian Wilson. The payoff pitch was over the plate, near the bottom of Dunn's kneecaps, and called strike three by home-plate umpire Tim Timmons.
"Very tough one," Nationals manager Manny Acta said about the call. "It looked like it was a little low for a guy like him, but it was bases loaded, 300th win on the line, a lot of things factored in."
That was the Nationals' best chance to get rookie Jordan Zimmermann, owner of two career wins, off the hook for his third loss of the season.
Zimmermann had given up 16 first-inning runs in his previous seven starts, but after the rain delay, he cruised for a 1-2-3 opening frame.
It was the second inning -- Zimmermann's lone imperfect one -- that proved to be his undoing. In the second, Fred Lewis singled, Travis Ishikawa doubled, and the two were brought around to score on a Juan Uribe groundout and Emmanuel Burriss single, respectively.
"He threw the ball very well," Acta said of Zimmermann. "He made a mistake giving up a hit on 0-2 against the eighth hitter with the pitcher on deck, but that's something that he's here to learn."
Johnson was perfect through 3 1/3 innings and did not yield a hit until the fifth, when Elijah Dukes led off with a single up the middle, shattering his bat in the process.
The next batter, Austin Kearns, walked. But a potential rally was foiled when a Ronnie Belliard rocket hit off the pitching mound, and second baseman Burriss made a diving stop to initiate a 4-6-3 double play.
While Johnson was aided with that defensive play, his infielders were responsible for the lone run that he surrendered.
With one out in the sixth inning, shortstop Edgar Renteria fielded an Alberto Gonzalez grounder and threw low to first for an error. Nationals first baseman Nick Johnson made them pay by doubling in the gap to score Gonzalez and cut the deficit in half.
Earlier in that inning, in what he called "a senior moment," the Big Unit dove to stop a ball hit by Anderson Hernandez. Johnson felt soreness in his left shoulder, which he landed on during the dive, and he had to depart following the completion of the inning.
The Nationals could not crack the Giants' relief pitchers, who held them scoreless for the remaining three innings.
Nationals reliever Joel Hanrahan struggled in the top of the ninth, yielding three runs on four hits without recording a single out, as the Giants extended their lead to 5-1 and let Johnson breathe easy again.
Wilson struck out the side in the bottom of the ninth, securing Johnson's big win. The Big Unit -- who returned from the training room in time to see Wilson's big strikeout of Dunn -- showed little emotion in the immediate aftermath of the game. Later, during his press conference, he could finally soak in what he had just accomplished.
"This is kind of a long-term thing that has been going on for 21 years," Johnson said. "And you finally get to this day and you know that if your team plays well and you pitch well, something can happen that's only happened  other times. I'm kind of at a loss for words."
Even Johnson's opponents had to tip their caps.
"We watched history today," Acta said. "He's probably going to be the last guy to do this. His longevity and everything he's done for the game paid off today."
Mark Selig is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.