Donald Trump said Brexit was "a sign of things to come" and "Brexit is going to be a wonderful thing for Britain".
He said: "You will have your own identity and you will be able to negotiate your own trade deals. Getting approvals from the group of European nations, the consortium as I called it, was very, very tough.
"But Brexit will be a fantastic and wonderful thing for Britain."
Donald Trump said he was working on a post-Brexit UK/US trade deal
Mrs May said a trade deal was being worked on between the two nations that would make commerce "quicker and easier" when Britain exits the Brussels bloc.
The Prime Minister said: "We are discussing how we can establish trade negotiations and lay the groundwork for UK/US agreements and see how we can work together.
"Trade between America and the UK is already worth £150b a year.
"It's improtant for the prosperity of both of our nations."
Theresa May and Donald Trump vowed to work together on military and economic issues
The pair vowed to team up to crush Islamic State terror, especially when it comes to cyber warfare.
Mrs May said: "Today we have spoken about how we can step up our efforts to tackle Daesh on cyber space.
"We have to defeat the ideology behind them and I'm sure we will discuss Syria and Russia."
She also confirmed Donald Trump had been asked to visit Britain and sources suggest he would meet with the Queen during his proposed visit.
Donald Trump criticised the EU for being too slow when it comes to trade deals
The Prime Minister was asked where she disagreed with US President Donald Trump and how he reacted to it during their hour long conversation in the White House.
She said: "There will be times when we disagree but I'm clear there are many issues on which the UK and United States stand alongside each other.
"We are in agreement on several areas where we can build an even stronger special relationship."
President Trump reaffirmed his belief that waterboarding works and added that he was remaining neutral with regard to Vladimir Putin, saying he "didn't know the gentlemen".
He vowed to represent his people "very forcefully" and said he would happily tackle Islamic State terror alongside Mr Putin's troops.
Theresa May and Donald Trump spoke about Putin, ISIS and Brexit in the White House conference
Mrs May jetted in to America yesterday to speak at the Congressional Republican Retreat.
Mr Trump hosted Mrs May at the White House today and it is thought the pair have thrashed out a post-Brexit trade deal.
Her speech, which reaffirmed the "special relationship" between Britain and America, was greeted with a standing ovation.
May meets Trump: Historic first meeting in pictures
Fri, January 27, 2017
The two leaders will spend about an hour in face-to-face talks in the Oval Office, where President Trump has restored a bust of Winston Churchill removed by predecessor Barack Obama.
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President Trump Meets With British PM Theresa May At The White House
The Prime Minister outlined her plans for Britain to further its role as a world leader post-Brexit, she said the UK would work with America to crush Islamic State extremism and Mrs May also issued a warning over America's relationship with Russia.
She said President Trump should "beware" Vladimir Putin but should not shy away from "engaging with him frm a position of strength".