It's a huge embarrassment for May, who called the early election with the aim of shoring up support for the Conservatives ahead of the country's crucial Brexit negotiations.
The Downing St. resignations came as May worked to fill jobs in her minority government and replace ministers who lost their seats on Thursday.
Earlier, Brussels' chief negotiator Michel Barnier has said the talks would begin when Britain was ready, suggesting he would consider a short delay.
SIEGEL: The Conservatives evidently plan to govern with the Democratic Unionists, the dominant Protestant party in Northern Ireland providing the few votes in the House of Commons that they'll need to have a working majority.
However, Mrs May insisted that, as the leader of the largest party in the new parliament, she had a duty to act in the "national interest".
Before her defeat, May said she wanted to negotiate the divorce and the future trading relationship with the European Union before Britain leaves in March 2019, followed by what she calls a phased implementation process to give business time to prepare for the impact of the divorce.
Davidson also said she had received reassurances from May that the party's deal with the DUP would not involve a rollback of gay rights.
In the late stages of the campaign, Britain was hit by two Islamist militant attacks that killed 30 people in Manchester and London, temporarily shifting the focus onto security issues. "That's what we will deliver. now let's get to work".
However she has only until 2.30pm Tuesday to make that promise a reality, when she is likely to face a vote of confidence in the reconvened new parliament where the Conservatives are a minority after a disastrous election ended with a hung parliament. A formal agreement is yet to be reached.
The DUP said the election outcome had bolstered the union, but Sinn Fein leader Gerry Adams said the collapse of Northern Ireland's middle-ground parties had made a referendum on Irish unity inevitable. "I think the only thing that political commentators can agree on is that we have uncertainty right now and nobody has any clue what shape this negotiation is going to take".
The move has been slammed as a "coalition of chaos" by Lib Dem leader Tim Farron, who condemned Mrs May for putting her "party before her country".
Writing in The Times she said: "Mrs May condoned their behaviour and turned a blind eye or didn't understand how destructive they both were".
"If she has an ounce of self respect, she will resign".
Notable women MPs who retained their seats include May, Conservative Home Secretary Amber Rudd - who kept her seat by a razor-thin margin after several recounts - Green Party leader Caroline Lucas, who retains her party's sole seat in Brighton, and SNP's Mhairi Black, who became the UK's youngest MP when she won her seat in 2015 at the age of 20. Her Labour rival Jeremy Corbyn said she should step down.
And for more on politics in the United Kingdom we turn now to George Parker, a political editor of the Financial Times. Labour stunned even its own supporters by taking enough seats from the Conservatives to deny them a majority.
Scottish Conservative leader Ruth Davidson, who is gay, was among the first to express disquiet over a deal with the ultra-conservative DUP.
"Theresa May arrogantly gambled with our Brexit and blew it", said a spokesman for the Leave.EU pro-Brexit campaign.
Sky's Northern Ireland correspondent David Blevins said: "The DUP have a very delicate balance to strike between how they use this power they suddenly find themselves with, and how they ensure that anything they demand does not derail efforts to restore devolution here".
"With the potential of that direct rule being imposed of course by a Tory government shored up by the DUP".