"What the country needs more than ever is certainty," the Conservative leader said, as a political crisis brewed around her.
Sterling plunged against the dollar and the euro as even more uncertainty fed into the complex Brexit process, and European leaders bluntly reminded May that the clock was ticking.
May, who became prime minister after the June 2016 referendum on leaving the EU, had called the election three years early in a bid to strengthen her hand in the looming Brexit negotiations.
But in a catastrophic setback, the bet failed and she lost her overall majority.
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The centre-right Conservatives found themselves eight short of the coveted 326-seat mark after the Labour Party, led by socialist stalwart Jeremy Corbyn, scored hefty gains.
May reached out to Northern Ireland's Democratic Unionist Party (DUP), which won 10 seats, to forge a working majority.
The Conservatives and the DUP, which is socially conservative and backs Brexit, are expected to work together on a vote-by-vote basis rather than enter a formal alliance.
May's top ministers, including finance minister Philip Hammond, foreign minister Boris Johnson and Brexit minister David Davis, will remain in their jobs.
In a statement outside Downing Street, the 60-year-old premier promised to "fulfil the promise of Brexit".
"It is clear that only the Conservative and Unionist Party has the legitimacy and ability to provide that," she claimed. "This will allow us to come together as a country and channel our energies towards a successful Brexit deal."
But European Council President Donald Tusk warned there was "no time to lose" in starting the negotiations.
European Commission chief Jean-Claude Juncker added he hoped there would not be "further delay" in the talks that "we are desperately waiting for".
May confirmed she intended to start the Brexit talks on June 19 as planned, promising to "get to work".
She launched the two-year countdown to Britain's exit from the EU on March 29 before announcing snap elections less than three weeks later, causing precious haggling time to be lost.
Watch: Theresa May on UK election
Diplomatic veterans say the Brexit process is as titanic in scale as it is historically unprecedented.
It requires the unwinding of a four-decade relationship with Europe - with the risk that Britain, without a deal, could find itself locked out of the lucrative single EU market.
Tusk spelt out the problem on Twitter: "We don't know when Brexit talks start. We know when they must end. Do your best to avoid a 'no deal' as result of 'no negotiations'."
EU budget commissioner Guenther Oettinger said May was now likely to be a "weak" partner.
"The British need to negotiate their exit but with a weak negotiating partner, there is a danger that the talks are bad for both parties," he told German radio.
'Lost confidence'
May was deemed on track for an overall majority in the triple digits when she announced the elections on April 18.
But she came under fire for a lacklustre, soundbite-driven performance on the campaign trail and a damaging U-turn on healthcare policy.
Both were skilfully exploited by Corbyn, who also found the upper hand after two terror attacks threw the spotlight on May's record in her former job as interior minister.
The vicar's daughter presented herself as a "strong and stable leader" but is now facing calls to resign after throwing away her party's majority.
Corbyn bitingly told May to quit, saying she had "lost votes, lost support and lost confidence".
Watch: SBS chief international correspondent Brett Mason reports from Downing Street
"I don't believe personally that Theresa May will remain as our prime minister indefinitely," said Conservative lawmaker Heidi Allen.
"In my view it may well just be a period of transition," she told LBC radio.
Former Conservative minister Anna Soubry added that the premier "is in a very difficult place... she now has to obviously consider her position".
"Mayhem" screamed the headline on the front page of The Sun, Britain's biggest selling newspaper, which normally backs the Conservatives.
The London Evening Standard, edited by former finance minister George Osborne who was sacked by May, splashed with a photo of her under the headline "Queen of Denial".
Final humiliation
With the results in from all 650 constituencies, the Conservatives won 318 seats - down from 331 at the 2015 election - while Labour was on 262, up from 229.
In a final humiliation, the Conservatives lost the safe west London seat of Kensington, Britain's wealthiest constituency, to Labour by a mere 20 votes after a campaign that targeted the incumbent MP for her pro-Brexit stance.
In a night that redrew the political landscape once again, the UK Independence Party (UKIP) - instrumental in campaigning for Brexit - lost millions of voters, prompting its leader Paul Nuttall to quit.
The picture was different in Scotland, where Conservatives scored gains against the Scottish National Party of Nicola Sturgeon.
The SNP, which had called for a new independence referendum, lost 21 of the 56 seats it won in 2015, losing voters worried by the prospect of a second divisive plebiscite.
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.
French President Emmanuel Macron meanwhile said he was "pleased that she would continue to be a close partner" and agreed their countries' "strong friendship... was important and would endure." He invited her to visit France at the earliest opportunity.
