Britain's transition of power unfolded with startling speed since the June 23 referendum on European Union membership.
Before that, in the coalition government of 2010-15, she worked as a junior minister under the Lib Dem Energy Secretary Ed Davey, who usually has nothing but contemptuous words for his former Tory colleagues but sung her praises to me in an interview a year ago for her intelligence and ability to grasp a brief in detail.
"As we leave for the last time, my only wish is continued success for this great country that I love so very much".
"From the introduction of same sex marriage to taking people on low wages out of income tax altogether, David Cameron has led a one nation government and it is in that spirit that I also plan to lead", she said.
"When it comes to opportunity we won't entrench the advantages of the fortunate few".
The Daily Telegraph said in an editorial that Cameron leaves behind a stronger country, a thriving economy and more chances to get on in life. But The Sun said Cameron had been undone by his Olympian overconfidence, while the Guardian called him a prime minister of broken promises..
Mrs May wasted no time appointing her new cabinet with prominent Brexiteer Mayor Boris Johnson appointed Foreign Secretary and leading Remainer Philip Hammond replacing George Osborne as Chancellor.
Boris Johnson arrives in 10 Downing Street on Wednesday.
German Foreign Minister Frank-Walter Steinmeier meanwhile gave a speech criticizing those politicians who campaigned for Brexit only to "bolt and not take responsibility...."
May, 59, has been Britain's home secretary, in charge of immigration and law and order, for the past six years. He had campaigned to remain in the EU.
The 54-year-old has previously held a string of senior positions in government.
Cameron said being prime minister has been the "greatest honour of my life" in his final remarks. Soon after, the palace confirmed that Cameron had "tendered his resignation as prime minister and First Lord of the Treasury, which her majesty was graciously pleased to accept".
Voters ignored Cameron's warnings that going it alone would be a "leap in the dark" that would bring on a self-inflicted recession and Cameron himself underestimated public anger at the establishment, exacerbated by his government's spending cuts.
Scotland bucked the national trend for the referendum, with 62% voting to remain.
On a more serious note, Cameron said he cared passionately about the United Kingdom, adding that "we need to make sure as we leave the European Union, how we keep the benefits of the common travel area".
Not all believe her. As May spoke in front of her new residence, a small band of pro-Brexit demonstrators down the street chanted "Theresa May, don't delay!" But Tony Travers of the London School of Economics said May probably would not rush to trigger Article 50 of the European Union constitution, which starts a two-year countdown to a departure from Europe.
The Oxford University-educated daughter of a Church of England vicar, May is highly regarded among European officials, who say she knows Brussels well and is always well prepared.
I dont detect Theresa May being an impulsive person, Travers said.
Cameron used his final day in office to try to stake a claim on a legacy beyond his status as the prime minister who - by gambling on a referendum - took Britain out of the EU.
"I will miss the roar of the crowd, Cameron told Parliament".
Calling an election could strengthen her mandate - prime ministers holding office without winning a general election tend to have more limited clout.
"Nothing is really impossible if you put your mind to it", Cameron said.
He said that he hoped they would meet "in the near future".