The English FA had been in talks with his representatives for several months, but the 21-year-old has chosen to go with his heart and it will respect his decision.
And Jonathan Norcroft of the Times reports McLeish has persuaded the Lancaster-born midfielder - who qualifies for Scotland through Glasgow-born dad - to switch his allegiance to the Tartan Army after England were also keen to make him their own and is expected to be selected for this month's friendlies with Costa Rica and Hungary.
McTominay has not yet represented Scotland at any level but attended training camps with the country's younger sides and previously admitted: "I'm probably more Scottish than I am English".
Former Scotland worldwide James McFadden believes the news is like making a major signing.
His promotion to the Manchester United first team begged the question as to where his worldwide future lay, however. "I can't confirm because I have no information about that and my advice to him is not about national teams, is not about that".More news: Powell says Fed believes rates can go up gradually
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"We have to realise he's a young lad, he's playing for Manchester United and he's going to have a great future, but at the same time it's a totally different game playing for Scotland. If he's got that, it'd be very hard for him to play for England".
I think if they're not on the radar for the England team then it's much easier.
Fletcher who played in Scotland's final two World Cup qualifiers against Slovakia and Slovenia last October, is waiting to discover whether he will remain part of new Scotland boss McLeish's plans.
Mourinho added: "I think the people around him are the right people".
"I'm sure he would have met with Alex and straight away got to see what kind of guy he is".