Houston beat Sporting Kansas City 2-0 earlier on Sunday to win the Eastern Conference title in a fiercely-contested encounter in Kansas City, but the Dynamo were deserved winners after grabbing control of the game from the early stages.
Landon Donovan gave Galaxy the lead with a 23rd minute penalty and after Alvaro Saborio equalized two minutes later, Beckham's curling cross found the head of Mike Magee to put them 2-1 ahead in the 58th minute.
With Real pushing for the equaliser, Irishman Keane took advantage of the gaps at the back and sealed the win in the 68th minute.
Real, with defender Jamison Olave back in the starting line-up after a picking up a quad injury in the Western semi-finals, had their chances with Kyle Beckerman hitting the post just before half-time.
Galaxy, who lost to Real in the 2009 MLS Cup, had the best regular season record and cruised past the New York Red Bulls in the Major League Soccer conference semi-finals.
GALAXY FOCUSED
Beckham could be playing his last season in Los Angeles with his lucrative five-year contract set to expire at the end of this campaign but he said he was focusing on the Cup, not his future.
"I'm a Galaxy player right now. I'm 36-years-old and I still enjoy soccer," he told reporters. "Right now we're focused on the cup. We don't want to get too carried away. We did that a couple years ago and paid for it."
In Kansas City, Andre Hainault scrambled Houston in front in the 53rd minute after Jermaine Taylor got on the end of an Adam Moffat free-kick.
Moments later, Houston had a huge chance to double the advantage but Luiz Camargo hesitated badly in front of goal and the Sporting defense managed to clear.
Sporting, previously known as the Kansas City Wizards, renamed themselves this season to coincide with their move to their own new stadium, Livestrong Sporting Park.
The venue was packed with a record 20,800 crowd hoping to see their team reach their first MLS final since 2004 but for all the support, coach Peter Vermes's team struggled to impose themselves.
Houston, who did not win in their opening 15 games on the road in the regular season, won the midfield battle despite losing key playmaker Brad Davis to injury in the 39th minute.
Davis has been the single most influential figure in Houston's run to the final and they will now be hoping he is fit for the game in Los Angeles on November 20.
"It is too early to say," said Davis of his chances of making that game. "I just couldn't be more proud of the spirit we showed, it was thoroughly deserved."
Dynamo, MLS champions in 2006 and 2007, made sure of the victory in the 87th minute when Honduran forward Carlo Costly broke clear and blasted home confidently.
Sporting threw everything forward in the latter stages but despite a few penalty area scrambles, they created little in the way of real chances and paid the price for their lack of inventiveness against
