"I predicted Brexit. I was cutting a ribbon for the opening of Turnberry - you know they totally did a whole renovation, it is lovely - the day before the Brexit vote", Trump said.
Their joint press conference came just hours after Trump had rubbished May's Brexit plans and warned that it will "kill" chances of a trade deal between the United States and Britain.
"I think she found it maybe too brutal", Trump said of the suggestions he made to May regarding Brexit.
Some Conservative lawmakers have cast May's "business-friendly" Brexit plan as a betrayal that would leave Britain too close to the European Union and warned that she might face a leadership challenge.
He is planning to meet with Russian President Vladimir Putin next week.
While he spoke highly of Johnson, he also heaped praise on May, calling her an "incredible woman" who is "doing a fantastic job". "We wanted to embarrass him and I think we have done that today". And he warned that Britain's trade relationship with the US could be in jeopardy if May follows through with her "soft" Brexit blueprint. However, under the blue skies of the Chequers' garden, he said their relationship was "the highest level of special".
In an interview with The Sun published hours before the two leaders held talks, Mr Trump castigates Mrs May for ignoring his advice by opting for a soft Brexit strategy.
"I mean that. And I must say, I have gotten to know Theresa May much better over the last two days than I have known her over the last year-and-a-half".
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Meanwhile, Arsenal will be looking to at least break back into the top four next season after they finished sixth and without a trophy last term.
"I actually told Theresa May how to do it but she didn't agree, she didn't listen to me", Trump added.
"He does listen to her, he does have, I think, a genuine respect for the United Kingdom and for the office that she holds". "She's a total professional, because when I saw her this morning I said, 'I want to apologize, because I said such good things about you"'.
Trump also claimed Khan - with whom he has clashed in the past - has "done a very bad job on terrorism". If Trump said it, it's not clear when or where.
He defended his decision to allow the giant Trump baby inflatable to fly over London, saying: "The idea that we limit the right to protest because it might cause offence to a foreign leader is a slippery slope". I get along with her very nicely.
President Donald Trump and Queen Elizabeth II inspecting a Guard of Honour.
"Whichever way you cut it, it's a pretty dramatic Friday 13th", Farage said, commenting on some of the frantic reactions seen in Britain since the US leader's arrival to the island nation.