With one towering swing, the 36-year-old first baseman shattered 19 years of frustration, and the Giants crushed the Rockies, 9-2, on Wednesday at Coors Field to end a five-game losing streak.
With his homer, McClain became the oldest position player to hit his first big-league home run since Cincinnati outfielder Bob Thurman, a veteran of the Negro Leagues, hit his first homer at age 37 on April 16, 1955, according to Elias Sports Bureau.
But he might as well have grounded out with the way his teammates reacted in the dugout.
"It was awesome," Barry Zito said. "He just kind of walked in and was like, 'All right, thanks, guys.' After that, we jumped up and started hugging him."
"Typical," said McClain, who has given the silent treatment hundreds of times without ever receiving it. "I kind of expected it, to be honest."
But after spending such a long time in the Minors, there were times when he thought about quitting.
No more 4 a.m. wakeup calls. No more crushed hopes. No more exhausting flights to cities Carmen Sandiego would never be caught hiding.
"There was definitely times I thought it would never happen," McClain said. "I don't know if I'll ever get called up in September, so it makes it a little harder to do if you're not up here."
Despite only making brief stints in the Majors, McClain was a star in the Minors and Japan. His numbers would humble Crash Davis. In nearly two decades, he accumulated 362 home runs and 1,280 RBIs.
But here was the most telling number of all, the one that kept McClain second-guessing himself and his family up at night: 30 Major League games with three different clubs.
"What am I doing?" he asked himself after each of the past three seasons.
But he was called up from Triple-A Fresno on Monday, and on Wednesday he got the answer he'd been looking for.
"I wanted to slow things down and take it all in," he said. "It was a sigh of relief. A lot of people were pulling for me."
McClain was just one of a number of unproven players in Wednesday's starting lineup. They came through in a big way, bashing Rockies starter Aaron Cook (16-9) for six runs on 10 hits in three innings.
Nate Schierholtz, who won an Olympic bronze medal with Team USA and was called up Tuesday, went 4-for-5 with two doubles and three runs scored. Travis Ishikawa also played a leading role in the rout, tallying three RBIs.
Along with his two-run home run, Sandoval also had a single and scored in the Giants' four-run third.
"They picked us up today, these kids," manager Bruce Bochy said.
In fact, the six Giants rookies combined for all nine runs, all nine RBIs and 13 of the 14 hits -- which, according to Elias, is only the second time in the last 50 years a group of rookies have had that prolific an offensive performance. Florida's rookies scored nine runs, drove in 10 and had 14 hits in a 16-5 win over the Mets in 2006.
After a bad outing against the Reds, Zito (9-16) rebounded to pitch one of his finest outings as a Giant. The left-hander gave up just two runs on four hits and struck out five in eight innings.
He complained of a "flat" changeup in his last outing, but everything was sharp Wednesday. He effectively mixed his fastball and curveball to frustrate the Rockies.
"That was probably my favorite game as a Giant," he said. "I stayed within myself and didn't worry about results or anything like that. I just kind of let things happen around me instead of trying to make things happen."
The same could be said about McClain his entire career. He worked and waited but never languished. He controlled what he could and never gave up. And finally, after 19 years, it all paid off.
"We were just hoping he would get one," Bochy said. "He's such a great guy and he has such a love for this game. That's why he continued to play it. I'm glad he got this opportunity."
So is McClain.
Jeff Birnbaum is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.