May, former home secretary, is Britain's second female prime minister after Margaret Thatcher, who ran the country between 1979 and 1990.
She promised to lead a "one-nation" government dedicated to social justice and making 'Britain a country that works for everyone'. And together we'll build a better Britain.
JUST three weeks since the country voted to leave the European Union, the country has a new Prime Minister.
She then posed for pictures with her husband Philip, laughing and waving for the cameras, before heading inside.
May traveled to Buckinhgam Palace where she officially accepted the invitation of the Queen to form a new government.
She also stood firm on her promise that Brexit means Brexit.
May, who had supported Britain's continued European Union membership, moved quickly to heal divisions sparked by the referendum by appointing leading "Leave" campaigner Johnson to a senior cabinet post.
She won the Conservative leadership election after her final rival dropped out, and has gone to the palace to formally assume the office of PM.
"I wish you every success in the task ahead", he added. "I think she's a cautious person, and the British political establishment needs to come to terms with this massive decision".
David Cameron leaves Downing Street today having been Prime Minister of the United Kingdom since May 2010 and Leader of the Conservative Party since December 2005. "I will miss the roar of the crowd, I will miss the barbs of the opposition, but I will be willing you on".
Russian President Vladimir Putin has sent telegrams to David Cameron and Theresa May on Wednesday, the Kremlin press service said.
Prime Minister Benjamin Netanyahu sent a letter of congratulation to May, the Prime Minister's Office said in a statement.
The Russian leader thanked Cameron for his cooperation when he was in charge of the government and expressed hope that his "vast political experience would be in demand in the future both in Great Britain and on the global level". After Cameron's final private audience with the Queen, the monarch said her farewell, again in private, to Cameron's family.
The removal vans were already parked outside Downing Street by the time he left the House of Commons with others waiting to move Mrs May in.
Speaking in a packed parliament, with lawmakers, media, aides and spectators jammed into every corner of the ornate debating chamber, Cameron answered questions with the air of a man looking forward to an afternoon off.
He said his government had cut the deficit, overseen economic growth and legalized same-sex marriage.