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.