In his first speech as Prime Minister, he promised to give the country “the leadership it deserves” and said he would meet the October 31 Brexit deadline – “no ifs or buts”.
Tory sources are quoted as describing his ruthless culling of May’s old cabinet – cutting away around half her ministers – as a “bloodbath”.
The first appointments to Mr Johnson’s new Cabinet were Sajid Javid as Chancellor and Priti Patel as Home Secretary with his war cabinet taking shape to take on the EU and secure Brexit.
Dominic Raab has also returned to the Government as Foreign secretary, replacing Jeremy Hunt, and was handed First Secretary of State, effectively making him Mr Johnson’s Deputy PM.
Johnson also welcomed Gavin Williamson back to Government as his education chief after he was sacked accused of leaking from the Nationa Security Council while Defence Secretary.
Brexiteer ringleader Jacob Rees-Mogg – an outspoken critic of May – has also handed a top position as Leader of the House of Commons.
Following his appointment, Mr Rees-Mogg said: ”The parliamentary arithmetic doesn’t change. But the thing to bear in mind is that the Article 50 act and the Withdrawal Act mean that the default position is we leave on October 31.
“And that would have to be changed to stop the law taking its course. So it’s not simply a matter of a parliamentary motion, Parliament would have to change the law and it’s hard to see how that would happen.”
And meanwhile, Stephen Barclay will remain in the hot potato job of Brexit Secretary, Matt Hancock will stay as Health Secretary, Amber Rudd stays at work and pensions, and Ben Wallace and Theresa Villiers have been given defence and environment respectively.
Boris’s old archenemy Michael Gove has been appointed Chancellor of the Duchy of Lancaster, longterm Boris backer Liz Truss has been appointed International Trade Secretary and new face Robert Jenrick will take over housing in the Cabinet.
Two other returning Cabinet ministers were also Andrea Leadsom as Secretary of State for Business, and Nicky Morgan as Secretary of State for Digital, Culture, Media and Sport.
Boris’s former leadership rivals James Cleverly and Esther McVey have also been given jobs as the Conservative Party chairman and as a minister at Ministry of Housing, Communities and Local Government.
And his younger brother Jo Johnson – who he doesn’t see eye-to-eye with over Brexit – has been appointed a minister at the Department for Business, Energy and Industrial Strategy and the Department for Education.
Downing Street also confirmed that Geoffrey Cox will remain Attorney General and Baroness Evans of Bowes Park will remain Leader of the House of Lords.
Robert Buckland was also handed a job as justice boss, with the rest of the Cabinet including Brandon Lewis, Alok Sharma, Grant Shapps, Alun Cairns, Julian Smith, Alister Jack, Oliver Dowden, Kwasi Kwarteng and Rishi Sunak.
“I have decided now is the time for the biggest challenge of all – to be a good dad”
The appointments came after Mr Johnson wielded the axe, with more than half of Mrs May’s Cabinet either quitting or being sacked as infighting continues amongst the Tories.
Mr Johnson’s rival Hunt in the Tory leadership race, left the Government after refusing to be demoted from foreign secretary – saying he will now focus on being a “good dad”.
Penny Mordaunt and Liam Fox, prominent backers of Mr Hunt, were among the first to be sacked as Mr Johnson carried out a radical reshaping of the Cabinet.
Other ministers shown the exit include Damian Hinds, Greg Clark, David Mundell, Karen Bradley, James Brokenshire and Mel Stride.
Before Mr Johnson even took office, Philip Hammond quit as chancellor, David Gauke resigned as justice secretary and Rory Stewart left his post as international development secretary.
David Lidington, Mrs May’s de facto deputy prime minister, left office at the same time as his boss while ailing transport boss Chris Grayling alos resigned.
Raab has said he is “hugely humbled” to be taking on his new role – adding: “Obviously we have got the challenges of Brexit. That is something we want as a country to rise to,” he said.
“Obviously, the Foreign Office, the message about the opportunities around the world and our confidence in our country and taking those opportunities with enthusiasm and optimism is something I can’t wait to get involved with.”
Patel said it was a “great honour” to be appointed Home Secretary, adding the role comes with “significant responsibilities”.
She told Sky News: “I will do everything in my power to keep our country safe, our people secure, and also to fight the scourge of crime that we see on our streets.
“I look forward to the challenges that now lie ahead.”
Mr Johnson’s first dramatic hours as Prime Minister began with a wide-ranging speech in Downing Street shortly after being asked to form a Government by the Queen.
Watched by girlfriend Carrie Symonds, Mr Johnson promised he would “change this country for the better” and vowed to prove the Brexit doubters wrong.
He said: “I am standing before you today, to tell you the British people, that those critics are wrong – the doubters, the doomsters, the gloomsters are going to get it wrong again.”
He predicted that “the people who bet against Britain are going to lose their shirts because we are going to restore trust in our democracy”.
He added: “And we are going to fulfil the repeated promises of Parliament to the people and come out of the EU on October 31, no ifs or buts.
“And we will do a new deal, a better deal that will maximise the opportunities of Brexit while allowing us to develop a new and exciting partnership with the rest of Europe based on free trade and mutual support.
“I have every confidence that in 99 days’ time we will have cracked it.”
He promised action to fix the social care crisis, make the streets safe and improve the NHS.
He said: “I will take personal responsibility for the change I want to see.
“Never mind the backstop, the buck stops here.”
On the issue of the Irish border – the main stumbling block in reaching a Brexit deal – Mr Johnson said he is “convinced” a solution could be found without checks at the border and without “that anti-democratic backstop”.
But he added: “It is of course vital at the same time that we prepare for the remote possibility that Brussels refuses any further to negotiate and we are forced to come out with no deal.”
Independent MP Nick Boles, who served as a Conservative minister under David Cameron, said: “The hard right has taken over the Conservative Party.
“Thatcherites, libertarians and No Deal Brexiters control it top to bottom. Liberal One Nation Conservatives have been ruthlessly culled.
“Only a few neutered captives are being kept on as window dressing.
“The takeover that started in local constituency parties is now complete.
“The Brexit Party has won the war without electing a single MP. Boris Johnson isn’t our new Prime Minister; Nigel Farage is.”
Labour criticised the appointment of “hardline” conservatives in Mr Johnson’s Cabinet.
Labour Party chairman Ian Lavery said: “Boris Johnson’s first act as Prime Minister has been to appoint a cabinet of hardline conservatives who will only represent the privileged few.
“A Chancellor who’s consistently called for more tax cuts for big corporations, Home and Education secretaries who were sacked for breaches of national security and a Foreign Secretary who doesn’t know the importance of our ports.
“This out-of-touch Cabinet pushed for nine years of damaging austerity, while demanding tax cuts for the super-rich and big corporations.
“We need a general election and a Labour government that will bring real change for the many, not the privileged few, which Johnson and his Cabinet represent.”
Allies of Mr Johnson played down the prospect of an early election or a pact with Nigel Farage’s Brexit Party – despite big talk coming from their mutual pal US President Donald Trump.
Farage said an election would be required to ensure Brexit can be delivered on time and if his party and the Tories agreed an alliance they could “smash” the Labour Party.
The US president suggested the Brexit Party leader would “work well with Boris” to do “some tremendous things”.
But an ally of Mr Johnson, Health Secretary Matt Hancock, played down fresh suggestions of a pact.
He told BBC Radio 4’s Today programme: “There is no way that we are going to have any kind of electoral pact with the Brexit Party and with Nigel Farage.”
Mrs May had earlier used her farewell address in Downing Street to urge Mr Johnson to secure a Brexit deal.
She said the “immediate priority” was “to complete our exit from the European Union in a way that works for the whole United Kingdom”.
The handover of power came on a dramatic day at Westminster which also saw Greenpeace protesters attempt to block Mr Johnson’s car on its way to Buckingham Palace.
It also saw European Council president Donald Tusk call for Mr Johnson to set out a detailed plan for Brexit.
And elsewhere Mr Johnson appointed controversial Vote Leave mastermind Dominic Cummings in an advisory role.
In his Downing Street address, Mr Johnson was keen to set out a domestic agenda in order to ensure his term in office is not defined by Brexit.
He confirmed his campaign pledge to put another 20,000 police on the streets, he said work would start this week on 20 new hospital upgrades, and he promised to “fix the crisis in social care once and for all” with a plan to give every older person “the dignity and security they deserve”.
Acknowledging the divisions in the country, Mr Johnson said he would answer the pleas of the “forgotten people and the left behind towns”, with investment in new transport links and infrastructure.
He also hailed the “awesome foursome” of the four nations of the UK – England, Scotland, Wales and Northern Ireland – “who together are so much more than the sum of their parts”.
Mr Johnson faces a difficult task as he attempts to govern with a majority of just two and now a host of disaffected former ministers.
His answer appears to be reuniting many of the key players from the Vote Leave referendum campaign which secured victory for Brexit in 2016.
Ms Patel was a leading player in the campaign and returns to government after she was forced by Mrs May to resign as international development secretary over unauthorised contacts with Israeli officials.
Mr Cummings clashed with officials and politicians while he was an adviser to Michael Gove in the coalition government, but Mr Johnson clearly believes his forthright style will help steer Brexit through.
The appointment of the abrasive Vote Leave campaign director is controversial given that earlier this year he was found to be in contempt of Parliament for refusing to give evidence to a committee of MPs investigating “fake news”.