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 front of 86,432 spectators at the Pasadena Rose Bowl.
Mata twice went close early on before he set up Welbeck to fire in a spectacular opener from 25 yards.
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.
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.
Man Utd v LA Galaxy
three more goals arriving in the final seven minutes, James netting his second of the game on the rebound after Young initial strike had been saved.
from Welbeck's cross.
Just after the hour mark, 20 year old James side footed the fourth from Young low cross with Puma Suede Black Creepers
And in first half injury time, Rooney netted his second and United's third with a simple finish Puma Ignite Hi
Wayne Rooney, Reece James and Ashley Young all scored twice as Puma Fenty Shoes Men Manchester United began the Louis van Gaal era with a 7 0 win over LA Galaxy.
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.
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.
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 Puma Fenty Creepers Green
De Gea in goal.
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