Griffey, who less than an hour before the All-Star Home Run Derby began was still insisting he would skip this year's competition, won the glamour event by beating Cleveland's Jim Thome in the final.
Griffey reversed his decision after being booed by a Coors Field record crowd of 51,231 while accepting a trophy for being the All-Star Game's top vote getter, named on over 4.2 million ballots.
"I don't like to get booed. I don't think anybody does," said Griffey, who hit 19 homers in the three rounds and beat Thome 3-2 in the final round. "This is not a time to get booed, the All-Star Game. If they want to see me do the home run competition, the fans, there were four million reasons why I did it."
As for Mark McGwire, his performance was a surprise, too.
Baseball's home-run leader managed just four home runs and failed to get past the first round. McGwire came to Denver with 37 homers, and many figured if he didn't win the title, he would certainly test the laws of gravity 5,280 feet above sea level.
But except for a 510-foot shot to dead center _ the longest of the day _ McGwire looked more like a singles hitter than the game's most feared hitter.
"I like the ball middle in, and the pitches were middle away," McGwire said. "So, sorry everybody."
Thome, who choked last year in front of the hometown fans at Jacobs Field by failing to hit one homer, hit seven in the first round and eight in the second round.
"This was great just to be in it," he said. "I hope they ask me back."
Early Monday afternoon, Griffey was insisting he would pass up the chance to rocket balls through the thin Colorado air. He reluctantly entered last year's event in Cleveland despite getting just a few hours rest after the Mariners played a late Sunday night game.
With his Seattle Mariners cap turned around backwards, Griffey dug into the dirt as his name was announced to loud boos.
"I figured there would be a few, but not like that," he said.
But when he was introduced as the final AL entrant, he received a warm ovation, and by the time he walked to the plate for his first round, many in the crowd were standing.
After connecting eight times to advance, he slowly walked back to the AL dugout, tipped his cap to the fans and hugged his 4-year-old son, Trey.
Then it was McGwire's turn. He hit two weak grounders before driving one over the wall in straightaway center, hitting a sign just below the Rockpile section of seats. The announced estimated distance of 510 feet surpassed the unofficial stadium record, a 496-foot shot hit by Mike Piazza last year.
Thome, Baltimore's Rafael Palmeiro, Colorado's Vinny Castilla and Houston's Moises Alou all finished the first round with seven homers, but Alou was eliminated because he has less regular-season homers than the others.
Atlanta's Javy Lopez and Seattle's Alex Rodriguez had five homers each. Detroit's Damian Easley led off the event and managed just two homers, one more than the Braves' Chipper Jones.
Before Monday's workouts, McGwire, who along with Griffey and the Cubs' Sammy Sosa, will spend the second half chasing Roger Maris' record of 61 homers in a season, said he still can't believe he's mentioned along with some of the game's great long-ball hitters.
"It still blows me away, it really does," he said. "Considering that when I was a kid, all I wanted to do was pitch. It wasn't until my sophomore year in college that I turned into a hitter, never knowing I'd get a chance to go to the big leagues.
"The next thing you know, they're talking about my name along with Babe Ruth, Maris, Mantle down the line. It's overwhelming. I don't think it will really hit me until I'm retired."