The Galaxy beat two-time MLS champions Houston 2-0 at the Home Depot Centre, a Beckham free-kick in the first half of extra-time setting up the goal which broke the deadlock in a Western Conference Championship game halted for more than 30 minutes during regular time as the area south of Los Angeles was twice plunged into darkness by a power outage.
The England midfielder, who joined the Galaxy from Real Madrid in July 2007, had helped the LA team reach their first post-season since 2005 and he sent in a 104th minute free-kick that found its way to veteran defender Greg Berhalter to fire home.
Landon Donovan added a second from the penalty spot soon after and the Galaxy moved on to next weekend's MLS Cup final in Seattle against either Chicago Fire or Real Salt Lake, who will contest the Eastern Conference final later today.
Beckham praised his team-mates and Tim Leiweke, president and chief executive of Galaxy owners AEG, for putting the MLS team within reach of their first title since 2005.
"It's so important for us to do well, we've had so many disappointments over the last four years and Tim Leiweke and everybody have put so much behind this team, this club and this franchise and the players are doing them proud," Beckham said.
"We've got a great bunch of guys that are working hard for each other and it's been tough, so we're happy."