Danny Welbeck, who opened the scoring in the 13th minute, was also on target in United's first game of their pre-season tour of north America, played in front of 86,432 spectators at the Pasadena Rose Bowl.
Van Gaal opted to start the match with three centre-backs – Chris Smalling, Jonny Evans and Phil Jones – while Antonio Valencia and teenage debutant Luke Shaw were used as wingbacks, with first-choice keeper David De Gea in goal.
Another summer signing, Ander Herrera, also made his debut in midfield alongside Darren Fletcher while Juan Mata, Rooney and Welbeck started in a 3-5-2 formation.
Mata twice went close early on before he set up Welbeck to fire in a spectacular opener from 25 yards.
That was the way it remained until three minutes before the break when Rooney added a second from the penalty spot after AJ Delagarza had handled Valencia's cross into the box.
And in first-half injury time, Rooney netted his second and United's third with a simple finish from Welbeck's cross.
Van Gaal made nine changes at the break bringing on Anders Lindegaard, Rafael, Michael Keane, Tyler Blackett, James, Shinji Kagawa, Tom Cleverley, Nani and Ashley Young.
Just after the hour mark, 20-year-old James side-footed the fourth from Young’s low cross with three more goals arriving in the final seven minutes, James netting his second of the game on the rebound after Young’s initial strike had been saved.
It was 6-0 on 88 when Young collected Herrera's assist to round the keeper and score from a tight angle and the former Aston Villa winger completed the scoring two minutes later as he latched onto Rafael's pass.
United are back in action on Saturday night when they take on Roma in Denver.