The match ended 1-1 in normal time after Jesse Lingard's opener for United was cancelled out in the second half by Casemiro, who blasted home for Real from the penalty spot.
Real boss Zinedine Zidane decided to play his senior players in the first half of the club's opening match for the pre-season, with Gareth Bale, Karim Benzema, Luka Modric, Toni Kroos and Marcelo all starting.
Despite the firepower in the Los Blancos line-up, the first half was lacklustre, until an unlikely hero emerged for United.
When the ball was picked up by Anthony Martial on the left-wing on the stroke of half-time, it's safe to say nobody envisaged what was to come.
The much-maligned Frenchman, whose £35m price tag has seemingly placed an immovable weight on his shoulders, was suddenly relinquished of his insecurities to produce an utterly brilliant piece of play to set up United's first goal.
Stuttering slowly with the ball, Martial then jinked his way past Lucas Vasquez, then Dani Carvajal and finally Luka Modric, before having the presence of mind to lift his head and square the ball to Jesse Lingard who simply tapped the ball into the wide-open net.
Although it was a friendly, the smile on Martial's face showed just how much it meant for him.
The second-half saw Zidane change his entire starting XI, while United brought on several of their stars including Paul Pogba, Romelu Lukaku and David De Gea.
The French manager would have liked what he saw from his youngsters; they were committed, skilful, calm and intense. A bit too intense at times, with Oscar Rodriguez crashing into Ander Herrera, which caused the United midfielder to leave the match.
Remonstrations from Mourinho poured out, but the referee was not having any of it.
With 20 minutes remaining, Real were rewarded a penalty after a poor challenge from Victor Lindelof.
The Swede, who United signed from Benfica last month, got his timing all wrong when he brought down Theo Hernandez inside the box.
David De Gea was given no chance from the resulting penalty after Casemiro cannoned the ball into the top-corner.
The scores were tied after 90 minutes of play, however, the International Champions Cup format meant a penalty-shootout would take place.
Both sides would rather of finished the game after normal time after the shoot-out finished a comical 2-1.
Lindelof, McTominay and Martial all missed for United, only to be outdone by Real, who had misses from Theo Hernandez, Casemiro, Oscar and Kovacic.
The two clubs play again on August 8, but this time there will be silverware at stake as they face off for the European Super Cup.