Speaking outside 10 Downing Street, she said she would lead a “one nation” government that would tackle inequality and preserve Scotland’s place in the UK while negotiating the country’s exit from the European Union.
Just an hour earlier, David Cameron bade an emotional farewell to Downing Street alongside his wife, Samantha, and their three children.
The outgoing Prime Minister said it had been the “greatest honour” to lead his country and insisted he had left the country “much stronger”. He shared a hug with his family on the steps of No.10 before getting in to a car to Buckingham Palace to hand his resignation to the Queen. Mrs May, the second woman to become Prime Minister, immediately began assembling her Cabinet last night, appointing Phillip Hammond as Chancellor of the Exchequer and making the surprise announcement that Boris Johnson will be Foreign Secretary.
George Osborne, who had been tipped for the Foreign Office, instead joins Mr Cameron on the backbenches, amid claims that he was sacked.
Mr Johnson’s appointment represents a remarkable political comeback a fortnight after his own bid to become Prime Minister was left in ruins.
David Davis, the veteran Eurosceptic, will lead crucial Brexit talks with the European Union, heading up a new ministry dedicated to negotiating the UK’s exit deal.
Amber Rudd will become Home Secretary, while Michael Fallon remains in post as Defence Secretary. Brexiteer Liam Fox, who resigned from government in 2011, will head up a new international trade department.
The Prime Minister put preserving the Union at the heart of her plan for government as she made her first speech from Downing Street.
Mrs May stressed that the full name of the Tory party included the word “Unionist” and pledged to keep the country together after the shock waves of the Brexit vote.
She said: “That word ‘unionist’ is very important to me. It means we believe in the Union, the precious, precious bond between England, Scotland, Wales and Northern Ireland.”
She said it also meant something else, just as important.
“It means we believe in a union not just between the nations of the United Kingdom, but between all of our citizens, every one of us, whoever we are, and wherever we’re from.”
She pledged to create “a country that works for everyone”.
Her speech from a lectern outside No.10 came just minutes after she had accepted the Queen’s invitation to form a government following David Cameron’s resignation in the wake of the referendum.
Edinburgh South Labour MP Ian Murray said Mrs May was inheriting a government whose economic credibility was in tatters after Brexit, one which had seen one of the slowest economic recoveries ever and which continued to risk the future of the UK.
He said: “A change in Prime Minister will not fool people across Scotland. Despite what she may say, Theresa May’s record in government shows that she will continue with the discredited austerity policies that have done so much harm over the past six years.
“What we need is a government that can deliver for working people, not more of the same from a Conservative Party under new management.”
Scottish Green co-convener Patrick Harvie said Mrs May should be asked to give evidence to MSPs in her first few months in office to discuss her plans for government.
He said: “The task faced by the new Prime Minister will require her to work constructively with opposition parties and the devolved bodies, however voters in Scotland will be troubled by her record on issues from civil liberties to climate change, and from the ‘bedroom tax’ to intervention in Iraq and Syria.
“It’s critical that Theresa May respects the fact that Scotland voted overwhelmingly to remain in the European Union, and that she allows the Scottish Government to explore every way possible to ensure our continued EU membership.
“Scotland rejected the Tories at the election last year and in acknowledging this, I hope that the incoming Prime Minister is willing to come to the Scottish Parliament to set out her plans for government.”
Meanwhile, MPs have launched an inquiry into Scotland’s future relationship with the EU. The Scottish Affairs Committee will look at what options exist for Scotland to stay a member of the EU and how Brexit will affect Scottish devolution and Scotland’s funding settlement.
The inquiry will also examine what role Scotland will have in the process of the UK withdrawing from the EU, how Scottish interests will be represented in negotiations surrounding this, and what future relationship with the EU would be best for Scotland.
Committee chairman, SNP MP Pete Wishart, said: “In the referendum on the UK’s membership of the EU, 62 per cent of Scotland’s population voted to remain. We have launched this inquiry to look at what options are available to Scotland to continue to secure a relationship with the EU including examining whether Scotland can continue its membership of the EU.
“As Brexit proposals start to emerge we will also examine what role Scotland should play in these discussions and what Brexit would mean for Scotland.”