Rooney scored twice and was joined on the scoresheet by England team-mates Danny Welbeck and Ashley Young, whilst young defender Reece James helped himself to a brace on his debut.
It was a dominant performance from the Reds at the Rose Bowl in what was new boss Louis van Gaal’s first game in charge of the Old Trafford club.
Phil Jones played in a three-man defence in United's victory
And Chris Smalling, Luke Shaw and Phil Jones featured in defence for United in what was a new look formation under the Dutchman, before England Under-21 international Michael Keane played the second half alongside former U19s defender Tyler Blackett whilst Tom Cleverley featured in midfield.
Welbeck got the scoring underway with the goal of the evening, a well-struck shot from 25 yards which cannoned home off the post to give Galaxy ‘keeper Jaime Penedo no chance.
The second arrived just before half time, when Rooney slotted home his first of the evening from the penalty spot after Antonio Valencia had been fouled.
Rooney’s second came soon after when he swept home from close range following a defensive slip from Tommy Meyer in stoppage time.
Van Gaal made nine changes at the break and it didn’t take long for United to re-adjust, with Young picking out James in the area for the full-back to strike home.
James, younger brother of former England U19s captain and Leicester City midfielder Matty James, then added his second of the game in the 84th minute when Penedo spilled Young's shot to leave the 20-year-old with a simple finish.
It was left to Young to finish the scoring with a late double, rounding Peneda to score his first after a pass from summer signing Ander Herrera before adding his second in stoppage time from a Rafael pass.
"The result always matters, with such a result you get confidence," said van Gaal after the game, which attracted over 86,000 fans in California.
"It is fantastic how they have performed today.
"When you see us train, you can expect something but not 7-0. It was a surprise, but they were beautiful goals and beautiful attacks."
The United squad will now head to Denver where they will play Roma on Saturday.
