He said that May's new, softer approach to Brexit - which would leave Britain sticking to a "common rulebook" with Brussels on goods - would kill off the chances of a much sought-after USA -U.K. trade deal. "I don't think so", Trump said.
He criticised Mrs May for the way she handled negotiations over Brexit negotiations, saying "she didn't listen to me" and that the deal she is pursuing "is not what the people voted on" and will affect trade with the United States "in a negative way".
Trump will be hosted at the Chequers, Windsor Castle and Scotland.
"I think allowing millions and millions of people to come into Europe is very, very sad", he said.
In the interview, Trump said Johnson has "got what it takes and I think he has got the right attitude to be a great prime minister".
Irish Prime Minister Leo Varadkar on Friday described the British government's white paper on Brexit as an evolution of its position, but not a solution.
British Twitter users hoped Prime Minister Theresa May would invoke Hugh Grant in the movie Love Actually and push back harder against President Donald Trump than she seems to have so far in their negotiations.
Claiming Khan has done a "very bad job on terrorism", as well as on crime.
This has infuriated fervent Brexit supporters, who think it would limit Britain's ability to strike new trade deals around the world. "Just make sure you can trade with us, that's all that matters", he said to May. "He has done a awful job".
Mr Trump added: "I have a great relationship with Angela Merkel". Other protests are taking place across the United Kingdom on Friday and Saturday.
Donald Trump's visit to London didn't exactly go as planned on Friday, July 13.
Trump told May that a year ago, the USA lost $151 billion in trade with the EU.
"I fear that this has become a bad relationship-a relationship based on the president taking exactly what he wants and casually ignoring all those things that really matter to, um, Britain", Grant says in the film. "I think he has done a bad job on crime, if you look, all of the frightful things going on there, with all of the crime that is being brought in".
Hundreds gathered under a large balloon depicting Trump as an angry baby while others shouted and held signs aspiring to show the president he wasn't welcome.
Trump expressed his admiration for Johnson in an interview with The Sun, published just hours before a scheduled meeting with Mrs May at her Chequers residence.
After calling Trump's attack on him "preposterous", London Mayor Sadiq Khan-who initially rejected organizers' efforts to fly the Trump Baby blimp-explained why he ultimately made a decision to allow the balloon to fly over London.