She became prime minister the month after the United Kingdom voted in June 2016 to leave the European Union, and her premiership has been consumed by the attempt to deliver on that verdict. Multiple media reports said she intends to resign as leader of the Conservative Party within weeks but continue as caretaker prime minister until the party elects a new leader.
The Tory leadership contest will be a packed race.
Was it the time she arrived in Brussels for a high stakes meeting with German Chancellor Angela Merkel, only to momentarily find herself trapped inside her auto while the world's media looked on?
Mrs May's decision to finally name the date for her resignation came after a bitter backlash against her last effort to get a Brexit deal through Parliament.
Dutch Prime Minister Mark Rutte tweeted that the "agreement reached between the European Union and the United Kingdom for an ordered Brexit remains on the table".
European Union spokesperson Mina Andreeva said Juncker watched May's announcement that she will step down "with emotion" and it was "without personal joy".
"We revise up our probability of "no deal". not because this Parliament (or indeed the next) is likely to coalesce in favour of its pursuit, but because the recent performance of the Brexit Party and the Eurosceptic credentials of the next Prime Minister may strengthen the case for including "no deal" on the ballot in a second referendum to unlock the impasse".
But, despite pledges, she has failed to reduce net migration to the tens of thousands she promised and a much-delayed social care green paper has still not been published. The EU has refused to renegotiate the withdrawal agreement and a majority of British MPs don't want the country to leave without a deal.
"I will shortly leave the job that has been the honour of my life to hold", she told reporters outside her Downing Street official residence. " But she soon sided with the hardliners agitating for a harder Brexit, egged on by the frenzied editors of Britain's mass-market tabloids".
The former Work and Pensions Secretary said she believes the next leader should be someone who "believes in Brexit" as she hinted at a leadership bid.
With Brexiteer Boris Johnson the current favourite to replace her, and former Brexit secretary Dominic Raab his nearest contender, Mrs May warned against a hardline approach, saying a consensus was necessary.
It is now unclear what will happen to the legislation but it seems likely that any new leader will seek a renegotiation.
He has repeatedly said Britain should not fear a so-called no-deal Brexit. Fill in the form below to give us your thoughts.
He is widely seen as the Conservatives' answer to the surging Brexit Party of anti-EU populist Nigel Farage.
British Jews thanked Prime Minister Theresa May for being a "true friend" to their community following her announcement that she is stepping down. "The second female prime minister, but certainly not the last".
"He will equally respect and establish working relations with any new prime minister, whomever they may be, without stopping his conversations with Prime Minister May".
And although Brexit has overshadowed her time in office, analysis by the Institute for Government shows that the vast majority of parliamentary time has actually been spent discussing matters other than leaving the European Union.