The young keeper came on at half-time for Anders Lindegaard as Sir Alex Ferguson remained true to his word in giving match-time to every member of the DHL tour squad.
Although Johnstone had a quiet time of things as United dominated in a 1-0 victory, it was clear that the experience meant everything to him, despite skipper Rio Ferdinand's best efforts to distract him during the exclusive MUTV interview.
"Of course it's a proud moment for me and my family," he stated. "I got a text off my Dad in the morning saying I'd been working for this since I was 10 years old, which a lot of the team have.
"We've been together a long time and there were a lot of youngsters on the field."
Sir Alex commented, also on MUTV, about his England Under-19 international, who looks set to either be third-choice at Old Trafford next term or head out again on loan.
* Penalty shoot-out hero at Old Trafford in the Barclays Premier Reserve League play-off final against Aston Villa.
* Has captained England at Under-19 level and was No1 at the recent European Championship finals.
* Also represented his country at Under-16 and Under-18 level.
* Made nine appearances in the successful Reserves side last season.
* Spent some of last term on loan at Scunthorpe United.