The Lancaster-born player has gone with his heart and pledged allegiance to his father’s native land despite an offer to represent England.
Alex McLeish is now set to select the 21-year-old this week in his first squad since returning to the role of Scotland manager.
The Football Association had been in dialogue with McTominay’s representatives for several months and he was being considered for an England under-21 call-up. But McLeish has persuaded the central midfielder to come on board as he begins his reign with friendlies against Costa Rica and Hungary this month.
England manager Gareth Southgate could not guarantee a senior call-up in the build-up to the World Cup and it is understood the FA respect the player’s decision.
Speaking in September 2016, McTominay admitted he felt more Scottish than English.
“The majority of my family is Scottish,” he said. “My mum has got a little bit but my dad’s side is all Scottish.
“I’ve been to a few training camps with Scotland. I’m probably more Scottish than I am English but it doesn’t bother me really. Who will I support in the World Cup qualifiers? That’s a tough question.
“It probably depends where I am, if I’m at my grandad’s it would have to be Scotland. It’s a bit of both - I want them both to do well.”
McLeish is expected to announce his squad on Wednesday with McTominay’s decision softening the blow of Scott Brown’s international retirement.
Former Scotland international James McFadden believes the news is like making a major signing. McTominay has made 17 first-team appearances for United, starting in crucial games against Sevilla and Chelsea in recent weeks.
McFadden, who has held talks over joining McLeish’s coaching team,said: “His stature, how he goes about his game, he reminds me of Darren Fletcher, and if we can get someone who has been as good for Scotland as Darren Fletcher it’s a great acquisition.
“I know you don’t really sign players for the international team but England were interested in him, and it’s fantastic he’s chosen to play for Scotland. Not because he’s not good enough for England but because he wants to play for Scotland and that’s great to see.”