Angel Di Maria also scored for Real, his 15th-minute goal putting the Spanish giants on the path to a fourth win in as many years over the two-time defending Major League Soccer champions.
With the win, Real booked a clash with English Premiership club Everton in the winners' bracket of the tournament on Saturday at Dodger Stadium in Los Angeles.
On the undercard at the home of baseball's Dodgers, the Galaxy -- who had a spectacular goal from Jose Villarreal -- will take on Italian side Juventus in the consolation bracket.
The tournament, which also features Chelsea, Inter Milan, AC Milan and Valencia concludes in Miami on August 7.
Real Madrid took charge from the start, and the Galaxy couldn't match their speed and skill.
Cristiano Ronaldo hit the crossbar with a header in the sixth minute, and less than two minutes later Denis Cheryshev's shot was saved by Galaxy keeper Brian Rowe.
Real Madrid went ahead in the 15th when Francisco Alcaron launched a ball from deep in his own half and found Ronaldo, who eluded three defenders to deliver it to Di Maria who slotted it past Rowe.
Although he continued to test the Galaxy keeper, Ronaldo didn't score. He departed in the flurry of halftime substitutions that brought on Benzema, who scored just six minutes after coming on.
Los Angeles were without Landon Donovan and Omar Gonzalez, who played on Wednesday in the MLS All-Star Game against Italian club Roma, and without injured Robbie Keane.
However, they managed to pull back a goal in the 63rd minute when Villarreal settled an attempted clearance from Madrid's penalty area, turned and fired a curling left-footed shot inside the right post.
Benzema restored Real's two-goal edge in the 74th, heading in a cross from Di Maria.