Beckham assisted on Mike Magee's goal, then drew a penalty that Landon Donovan converted, and L.A. defeated Red Bull New York 2-1 at The Home Depot Center to win the Western Conference semifinal series on aggregate, 3-1.
Luke Rodgers scored inside 4 minutes as New York tied the series on aggregate, but Magee scored late in the first half to restore L.A.'s lead and Donovan was on target in the 75th for his league-record 18th career playoff goal.
L.A. (2-0-0) won the first game at New York, 1-0, and overcame a controversial post-match scrum that resulted in red cards to Juninho of the Galaxy and Rafa Marquez of the Red Bulls to reach its third straight conference final.
The Galaxy will host Real Salt Lake, which defeated Seattle Sounders FC 3-2 in the other Western semifinal series, on Sunday in the West final.
New York (1-2-0) defeated FC Dallas in the wild-card round, but was unable to reach its first conference final since 2008, when it reached its only MLS Cup.
The Red Bulls turned up the heat on the pitch inside 4 minutes, as Rodgers ran onto a pass from Thierry Henry and beat Galaxy goalkeeper Josh Saunders with a first-time shot to the bottom-right corner to tie the aggregate score at 1-1.
Then just off the restart, the post-game theatrics from the first game carried over when Rodgers charged at Beckham and attempted a slide tackle. A number of players came together, but no further incidents occurred.
Rodgers received a yellow card, and his temper remained high for the remainder of the opening half, as L.A. settled in and gained control following an injury to influential New York midfielder Teemu Tainio.
Beckham's long pass to the far post in the 18th set up Magee for a chance, but his pass into the middle of the box was just out of the reach of Robbie Keane, and the ball just squirted across the 6-yard box untouched.
Keane drilled the outside of the left post in the 19th, then L.A. got a scare, as Beckham went down to the ground with an apparent injury in the 32nd. He was on the sideline for a brief spell, then returned to the field.
Rodgers was lucky not to receive a second yellow, and resulting red, following a late challenge on Magee in the 37th, and the Galaxy continued to pressure to find the equalizer before halftime.
Chris Birchall, starting for the suspended Juninho, had a long shot spilled by Red Bulls goalie Frank Rost, who recovered to deny Keane on the follow up just before the 40-minute mark.
L.A.'s breakthrough came minutes later, as Beckham's corner kick from the left was met on the near side by Magee, who turned a header from 7 yards just under the crossbar for his second goal in as many games this postseason.
Beckham also set up Magee's goal in the first leg. Magee has four career goals in the postseason.
Chad Barrett forced another save from Rost in first-half stoppage time, as the Red Bulls escaped a sluggish end to the opening 45 minutes just one goal down on aggregate.
New York started off the second half like it did the first, only a goal didn't result as Rodgers was denied an easy finish when Donovan raced into the box to flick a cross off his shoelaces, and then Rodgers fired into the side-netting on the outside of the left post.
Henry earned a yellow in the 61st that would have kept him out of the final in the West, potentially leaving New York without its two stars after Marquez was suspended for the rest of the playoffs for his head-butt, punch and flop after the first leg.
And after a slow stretch, L.A. sealed the series when Beckham earned a penalty kick after a foul by New York's Roy Miller. Beckham was shoved in the back by Miller, who also kick around the side of the former England captain.
Donovan easily scored with a shot to the left post in the 75th, converting for the fifth time on as many penalties this season. Donovan's 18 playoff goals are in just 26 matches.
L.A. easily held on over the final 15 minutes and stoppage time to extend its unbeaten streak at home this season to 18 matches. The Galaxy did not lose at home during the regular season, and will not play on the road the rest of the postseason.
In addition to hosting the conference final, The Home Depot Center is the host of the MLS Cup final.