Mrs Thatcher would be “fighting like a lioness for the union”, the Mayor of London said in a lecture to the Centre for Policy Studies entitled What would Maggie do now?
She put “the ‘great’ back into Britain” after a century of decline, according to Mr Johnson.
She defended “the Falklanders’ right to self-determination” and would have never put up with today’s “Eurocrats trying to tell us what to do”.
But Britain “hardly needs” its warships any more because it has shifted to “soft power”, he said.
He also criticised the First Minister’s economic adviser, nobel laureate Joe Stiglitz, who is among those “on the left” who have failed to find an alternative to capitalism.
Mrs Thatcher would have ended “the madness” of the immigration system, according to Mr Johnson.
He predicted that Britain will be the most populous country in Europe by 2060 but said there is “plenty of depopulated space” in the north to put people in.
Mr Johnson said: “She would now be fighting like a lioness for the union, and she would comfortably see off Salmond, as she saw off so many smart alecs, because she would have instinctively identified the heart of the matter.
“This isn’t about whether or not the Scots will be £800 per year worse off per head. This is about the demolition of Britain, about taking the blue background from the union flag, lopping the top off the most successful political union in history.
“It would diminish both Scotland and England, and it would be no consolation to her that the loss of Britain, as a concept, might also mean the end of the BBC.”
The BBC did its best “to foment an uprising” at Mrs Thatcher’s funeral by playing Ding Dong the Witch is Dead, he said.
Mr Johnston mocked the tune in his defence of capitalism and called for “automatic knighthoods” for the top one per cent of earners who pay 30 per cent of Britain’s income tax.
He said: “No-one on the left, from Paul Krugman to Joe Stiglitz to Will Hutton, let alone Ed Miliband, has come up with any other way for an economy to operate except by capitalism.
“Ding dong! Marx is dead. Ding dong! communism’s dead. Ding dong! socialism’s dead! Ding dong! Clause Four is dead, and it is not coming back. Like it or not, the free market economy is the only show in town.”
He added: “Britain used to rule the world, almost literally. Of the 193 present members of the UN, we have conquered or at least invaded 171 - that is 90 per cent. In the period 1750 to 1865 we were by far the most politically and economically powerful country on earth.
“And then we were overtaken by America, and then by Germany, and then we had the world wars - and we ended up so relatively weakened that the ruling classes succumbed to a deep spiritual morosity that bordered on self-loathing.
“Thatcher changed all that. She put a stop to the talk of decline and she made it possible for people to speak without complete embarrassment of putting the ‘great’ back into Britain.
“She sent the navy halfway round the world on a spectacularly risky venture; and by the end of the year Galtieri was gone and the military junta was no more, and the principle of the Falklanders’ right to self-determination had been vindicated.”
He continued in his Wednesday-night speech: “What would she do about Europe? I don’t think she would pull out of the single market that she helped to create.
“I think she would recognise that there is a chance to get a better deal. It’s time to sort out the immigration system so that we end the madness.”
It is “time we generally persuaded the Eurocrats to stop trying to tell us what to do”, Mr Johnson said, insisting “Mrs Thatcher would never have put up with it”.
He concluded: “By 2060 we will have more people than Germany. I don’t know exactly where they will all go; though when I drive through the cities of the north I see plenty of depopulated space.
“By the middle of this century we will still have a crown and we will still have a union.”