"As we leave the European Union, we will forge a new, bold, positive role for ourselves in the world, and we will make Britain a country that works, not for the privileged few, but for every one of us".
Philip Hammond, the former Foreign Secretary, has been made the Chancellor of the Exchequer, Amber Rudd is Home Secretary, and Eurosceptic David Davis has been made the new Brexit secretary.
The biggest surprise of May's cabinet so far has been the appointment of Johnson, until recently seen as her top rival for prime minister.
Liam Fox, another senior Brexit supporter, was made International Trade Secretary, almost five years after stepping down as defence secretary during the coalition government.
May devoted most of her first hours as Britain's Prime Minister in naming the key members of the team that will have the task of implementing voters' decision to leave the European Union.
In phone conversations with the leaders of Germany, France and Ireland, who called to wish her well as Britain's second-ever woman prime minister, May underlined her commitment to a British exit, or Brexit.
May, who was a supporter of the "Remain" campaign emphasised on Monday that Britain will forge a bold global role outside the European Union and that "Brexit is Brexit and we are going to make a success of it".
But that future now belongs to May, and it involves instability, uncertainty and tough wrangling over Britain's departure from the European Union which she must now negotiate.
Russian President Vladimir Putin has thanked departing British Prime Minister David Cameron in a valedictory message and congratulated his successorTheresa May.
May - who became Conservative leader Monday after her sole contender for the job, Andrea Leadsom, dropped out of the race - is Britain's second female prime minister after Margaret Thatcher, who held office from 1979 to 1990.
The United States said it was looking forward to working with him. And together we'll build a better Britain.
Minutes after meeting the Queen in Buckingham Palace, she directly addressed hard-working people and told them: "We will prioritise not the wealthy, but you".
During the campaign leading up to the Brexit vote, Johnson "had lied a lot".
Russia's President Putin for his part sent a message to May confirming "his willingness to have a constructive dialogue and to collaborate with the British Prime Minister about current issues concerning bilateral relations and the worldwide agenda", the Kremlin said.
Congratulations flowed in from world leaders, including Russian President Vladimir Putin, and Ms. May spoke briefly with German Chancellor Angela Merkel after taking office.
Mr Kenny previously pleaded with European Union leaders to give Britain time to adjust to the Brexit vote, but he has hardened his stance.
Mrs May will then have an audience with the Queen, when she accepts the monarch's offer to form a new government and will, in keeping with tradition, "kiss hands" with the head of state.