The youngster has the choice to represent either England or Scotland, and it won't be official until he plays a competitive tie for either side.
But McTominay insists he has made his final decision thanks to his father, Sir Alex and United's current manager.
"When I was in the development centres I was in and around The Cliff (United's training ground)," McTominay told the 'Sunday Herald': "Alex [Ferguson] was there and he spoke to my dad.
"After the memorial service for the Munich Disaster earlier this year he pulled me aside and told me: "Make sure you do play for Scotland!"'
However, McTominay was still unsure of his decision and chose to turn to United head coach Jose Mourinho.
"I also spoke to my manager at Manchester United (Mourinho). He told me to go and do whatever I needed to do.
"His advice was to weigh up both sides and then go with whatever is in your heart. I had already made the decision so it was reasonably simple. I took my decision and then we took things from there.
"It stands you in real good stead. With Jose and Sir Alex it is the confidence they can give you. Their belief gives you a bit more confidence going into matches."