The England midfielder, who has been struggling to recover from an injury to his left ankle, was cheered loudly by the 27,000 fans at the Home Depot Center as he ran on to the pitch in place of striker Alan Gordon.
Although his new team were beaten 1-0 by the FA Cup holders, Beckham was delighted to launch his much-trumpeted American adventure by playing a late cameo role in front of a packed stadium that included several Hollywood stars.
"It was very emotional," the former England captain told reporters after 12 minutes of action that featured a few deft touches, a tumble as he tried to evade a tackle by Chelsea's Steve Sidwell and a late corner kick.
"The reaction to me if I even take off my top or kick the ball has been incredible, it made me feel a bit embarrassed at times. I really wanted to be out there tonight with all the attention. It is a great day for the club."
His problem ankle was another matter, though.
"I'm not fit and I haven't trained since I have been out here," Beckham said. "The ankle is swollen and we'll see how it is tomorrow. It is not going to go away for a week or so but it's making progress, it's not a serious injury."
Galaxy president and general manager Alexi Lalas told ESPN television: "It is a long-term process and we want to make sure David is 100 percent.
BECKHAM CHANCE
"We have games next week, next month and over the next five years. Everyone will get their chance to see David Beckham here and across the country."
Signed by the Galaxy from Real Madrid on a $32.5 million five-year deal, Beckham had been one of seven substitutes named for the match by Galaxy head coach Frank Yallop.
His pop star wife Victoria watched the action from a corporate suite accompanied by her actress friend Katie Holmes, wife of Tom Cruise.
Other celebrities in the crowd included Californian Governor Arnold Schwarzenegger, U.S. actresses Eva Longoria and Jennifer Love Hewitt and English actors John Hurt and Ray Winstone.
Beckham did not appear on the pitch when his Galaxy team mates initially appeared for their pre-match warmup, delaying his arrival until both teams had run out of the tunnel shortly before kickoff.
Accompanied by a security official, the former England captain then made his way to the Galaxy bench with the capacity crowd at the Home Depot Center roaring its approval.
After Beckham sat down next to his fellow substitutes, he was encircled by around 100 photographers desperate to record the moment.
Beckham twisted his ankle playing for his country against Estonia in a Euro 2008 qualifying match last month.
He aggravated it in his final appearance for Real 11 days later when his former team clinched the Spanish league title.
Saturday's game, a sell-out, was the Galaxy's second-highest grossing fixture since the MLS side hosted Real a year ago, with Beckham playing for the opposition.