Landon Donovan’s 72nd-minute goal, from a pass by Irish international Robbie Keane, was enough to secure the victory for the Galaxy.
Beckham, who won six English Premiership titles with Manchester United and a Spanish League title with Real Madrid, captured his first MLS championship in the final match of his five-year deal.
The triumph put the finishing touches to a season in which the Galaxy produced the best regular-season record in the league and were unbeaten at home.
The lone goal of the match came as Beckham headed a ball to Keane, who ran into the area and fed Donovan, who fired home.
Keane, who this month helped Ireland book their Euro 2012 berth, appeared to score in the 56th but the goal was ruled offside.
When the final whistle blew, Beckham wrapped his arms around Donovan and lifted him off his feet in celebration.
Donovan paid tribute to his “warrior-like” team-mate, saying Beckham injured a hamstring earlier in the week and was “playing on one leg.”
“During the match I had a little bit of discomfort,” admitted Beckham, who clearly lacked quickness around the pitch but still displayed his deft touch with corners and free kicks.
“He gutted it out tonight,” Galaxy coach Bruce Arena said. “My hat is off to David.”
Beckham himself was just happy to help deliver a title for the Galaxy, who brought him Stateside to great fanfare in 2007 in a move that raised the profile of MLS with US sports fans and opened the door for other marquee international players to follow.
“It’s about time,” said Beckham, who could yet have a sequel with the Galaxy if the team can persuade him to return after his contract expires at the end of December.
Beckham, who is also being wooed by French club Paris Saint-Germain, preferred to celebrate rather than speculate on his future.
“At the moment, I’m going to enjoy tonight,” Beckham said. “There’s a lot of options but I’m a Galaxy player. I’ve been a Galaxy player for the last five years and I’ve been proud of that,” added Beckham, who was serenaded by chants of “We want Beckham” after the final whistle.
Man of the match Donovan said it won’t be his goal that he remembers from the night.
“For the last month have been so determined to win — I didn’t care who scored, I didn’t care if it was an own-goal, I just wanted to win. Winning feels so good. The moment when the whistle blew is what I’ll remember.”