In a joint statement, Mrs May and Mr Juncker said their teams would press on to “explore the options in a positive spirit”.
The comments came after the Prime Minister travelled to Brussels to seek legal assurances on the Northern Ireland Brexit backstop she believes are needed to secure Parliamentary approval for her Withdrawal Deal.
The two plan to meet again before the end of February, when Mrs May faces a new test in Parliament where her minority government just lost three MPs.
With 37 days to go to a potential chaotic Brexit, officials were "seized of the tight timescale and the historic significance of setting the EU and the UK on a path to a deep and unique future partnership", the statement said.
Mrs May said afterwards progress had been made over the backstop but added time was running out to secure changes.
The statement said discussions had looked at "which guarantees could be given with regard to the backstop that underline once again its temporary nature and give the appropriate legal assurance to both sides".
Talks also covered "the role alternative arrangements could play in replacing the backstop in future".
It is understood that the talks could give Mrs May momentum for more talks with EU leaders during a summit with their peers in Egypt on February 24 to 25. She could then return to Parliament next week and offer MPs some progress before another vote.
But the EU has squarely refused demands to discard the backstop, saying it is essential to avoid customs controls on what would become a new EU-UK border between Ireland and Britain's historically troubled province of Northern Ireland.
The two leaders said they tasked the EU's chief negotiator Michel Barnier and Brexit Secretary Stephen Barclay "with considering the process the European Commission and the UK will follow".
The discussions looked at whether changes can be made to the Political Declaration on the future relationship between the UK and EU that are consistent with positions taken by both sides.
The PM and Mr Juncker agreed to talk again before the end of the month.
Mr Barclay and Advocate General Geoffrey Cox are visiting Brussels on Thursday to discuss “legal text” on the backstop with Mr Barnier.
Both Mr Barclay and British finance minister Philip Hammond have suggested the "alternative arrangements" to the backstop that Britain is pursuing could become part of the new EU-UK relationship after Brexit, not the formal divorce treaty.
After the meeting, Mrs May said: "I have underlined the need for us to see legally binding changes to the backstop that ensure that it cannot be indefinite.
"That's what is required if a deal is to pass the House of Commons.
"We have agreed that work to find a solution will continue at pace.
"Time is of the essence and it is in both our interests that when the UK leaves the EU it does so in an orderly way.
"So, we have made progress."
While "alternative arrangements" could mean a small step towards a compromise with the EU, frustrated EU diplomats said time was fast running out.
"They have until March 10, maybe March 15 at the latest," one EU diplomat said. "Otherwise they will be forced into a delay of Brexit, or crash out."
Many EU officials believe that, come what may, Britain will have to ask for a delay to the March 29 Brexit date, if only to give it time to pass further legislation ahead of its departure.
The PM believes that gaining legally binding assurances that the backstop will not extend indefinitely is the key to winning the support of MPs for her deal, and seeing off efforts to extend the negotiation period in a series of Commons votes expected on February 27.
The backstop arrangements would see the whole of the UK remain in a customs union with the EU and Northern Ireland following some single market rules until a wider trade deal is agreed, in order to prevent the need for checkpoints on the Irish border.
Additional reporting by agencies.