Mrs May confirmed Mr Trump was “100% behind Nato” despite the president’s recent comments calling the transatlantic alliance obsolete.
Both leaders said they would work to establish trade negotiation agreements.
Mrs May also said Mr Trump had accepted an invitation from the Queen for a state visit later this year.
The prime minister added that a trade agreement between the UK and US was “in the national interest in both our countries”.
Although the UK cannot begin to negotiate trade deals until it leaves the EU, Mr Trump has said he wants a “quick” deal after that.
When asked about Mr Trump’s scheduled phone call with Russian President Vladimir Putin on Saturday, the president played down any suggestion that he would lift US sanctions against the Kremlin.
“It’s very early to be talking about that,” he told reporters during a news conference.
He also said having a “great relationship” with countries like Russia and China “would be a positive, not a negative”.
Meanwhile, Mrs May stood firm with the European Union’s stance on sanctions against Russia.
“We have been very clear that we want to see the Minsk agreement fully implemented,” she said, adding that the sanctions would continue until that is achieved.
Mr Trump said he would also defer to retired general and Defense Secretary James Mattis on whether he would reinstate the use of waterboarding as an interrogation technique.
The president said Mr Mattis does not “necessarily believe” in waterboarding and other interrogation techniques, which critics view as torture.
“I don’t necessarily agree, but I would tell you that he will override because I’m giving him that power,” Mr Trump said of General Mattis.
“I happen to feel that it does work. I’ve been open about that for a long period of time. But I am going with our leaders.”