Receiving her first post-election calls from foreign leaders, May was congratulated by US President Donald Trump, who agreed with her on "continuing the close cooperation" between Britain and America, Downing Street said.
Britain's best-selling Sun newspaper said senior members of her party had vowed to get rid of May, but would wait at least six months because they were anxious that a leadership contest now could propel Labour leader Jeremy Corbyn into power.
Following the election, May faced backlash over the failure of the Conservative party, with a number of politicians - including Corbyn - suggesting that she has to resign.
Sen. Bernie Sanders congratulated Jeremy Corbyn and the Labour Party he leads for what's being called one of the most stunning upsets in United Kingdom political history.
But May, facing scorn for running a lacklustre campaign, was determined to hang on.
Her minority government looks set to be propped up by the Democratic Unionists (DUP).
Where the center-right Conservatives had hoped to take seats from the main opposition Labour Party, the reverse happened. Speaking after the outcome became clear, May acknowledged that the result was a disappointment. That raised the odds that an election called by May to provide "strong and stable government" would bring instability and the chance of yet another early election.
Conservative Prime Minister Theresa May is trying to reach an agreement with Northern Ireland's Democratic Unionist Party to form a minority government.
But a shock projection of the results, released at 10 p.m. local time on Thursday immediately after voting finished, showed May would win just 314 of the British parliament's 650 seats. But May has already attracted criticism for the decision because of the DUP's position on same-sex marriage and abortion rights.
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Even if it passes it would have to go back to the House for a vote before landing on Donald Trump's desk to be signed into law. Without massive taxpayer subsidies, millions of Americans will find their health insurance options curtailed or nonexistent.
One of the lead European Union negotiators, Guy Verhofstadt, criticized May on Twitter, writing: "Yet another own goal, after Cameron now May, will make already complex negotiations even more complicated". May had been expected to win comfortably.
The pro-European Liberal Democrats, who have campaigned for a second European Union referendum, were projected to increase their number of seats from nine, but their former leader Nick Clegg, who was also a deputy prime minister from 2010 to 2015, lost his seat.
In the Conservative Party, recriminations were immediate and stinging.
"What the country needs more than ever is certainty", she said.
Jeremy Corbyn could have been Britain's next Prime Minister - if it wasn't for the performance of the Conservatives in Scotland.
The poll was released after polls closed at 10 p.m. (2100GMT), ending an unsettled national election held in the shadow of three deadly attacks in as many months.
"Absolutely", Corbyn told Sunday Mirror newspaper in an interview.
"Politics has changed and this is people saying they have had quite enough".
Initially blind-sided by May's snap election call, and written off by many pollsters, Labour surged in the final weeks of the campaign.
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