United got their fourth of the night on 67 minutes when Martial burst forward and threaded a pass to Mkhitaryan who made no mistake with a confident low strike.
Martial made it 5-0 five minutes later, rifling home Timothy Fosu-Mensah’s pullback.
Galaxy meanwhile, who had failed to register a single attempt on goal in the first half, bagged two late consolation goals through Mexico international Giovani Dos Santos.
Mourinho said the game had been “an amazing training exercise.”
“That’s what we need,” he said. “They gave us two different teams they gave people with intensity, so it was good for us.”
Mourinho was also satisfied with an experimental formation which saw him play with a three-man central defence.
“We tried a system that is not our main system,” he said. “We were working a few days on this to try to make the players feel a new experience in a new position and I’m really happy happy with that.
“It’s something we have to keep working and keep improving in case during the season we decided to play with three at the back. Maybe we want to do it more times.”
United travel to Utah to face Real Salt Lake in the second game of their US tour on Monday.