Johnson and Trump are both critical of the UK PM's handling of Brexit, likening her plans as causing Britain to function like a "colony state" unable to make its own laws.
Just hours earlier at a lavish dinner to mark Trump's first visit to Britain as president, May made a direct pitch for a deal with Washington.
Speaking to reporters in Belgium after a fiery Nato Summit, Trump had described the United Kingdom as a "hot spot right now with a lot of resignations" and dismissed the prime minister's Chequers plan on the next stage of Brexit.
As Trump is expected to discuss a possible post-Brexit trade deal and other global issues with May, organizers of Friday's demonstrations say there is no shortage of issues that will inspire mass numbers of people to attend.
Brexit-backing MPs, including leading eurosceptic Conservative Jacob Rees-Mogg, will also seek to force her hand by tabling amendments to a trade bill being debated in the House of Commons next week. "I think they agree with me on immigration", he told reporters in Brussels.
In remarks that will likely further weaken May's already tenuous grip on her own government, Trump told The Sun, a British newspaper, in an interview published Friday (13 July) that her proposed "soft" post-Brexit relations with the European Union would scupper a United States trade deal.
"Once we have our agreement with the European Union in place, I'm confident that there will be the latitude and the ability to do a significant trade deal with the United States".
The comments were published on Thursday as May hosted an elaborate state dinner for the president and his wife Melania in honour of their arrival in the country.
The trip coincides with a tumultuous week for May after two senior ministers resigned in protest at her plans for trade with the European Union after Britain leaves next March.
"I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London", Mr Trump told the newspaper.
Large anti-Trump protests are expected to follow Trump throughout the day.
Protests are expected against Trump, whose policies - including a travel ban on a number of predominantly Muslim countries, detention of child migrants on the US-Mexico border and imposition of tariffs on European Union steel and aluminium exports - have all been criticised by the UK.
"The President likes and respects Prime Minister May very much".
"Boris Johnson's a friend of mine, he's been very nice to me, very supportive and maybe I'll speak to him when I get over there".
May and Trump are scheduled to hold talks and a joint news conference on Friday.
But trip planners may go out of their way to shield Trump from viewing another aspect of the greeting: an oversized balloon depicting the president as an angry baby in a diaper that will be flown from London's Parliament Square during what are expected to be massive gatherings of protesters opposed to Trump's presence.
A high fence has been erected around the US ambassador's central London residence where Trump will spend Thursday night and the embassy has sent out an alert warning Americans in London to keep a low profile in case protests turn violent.
From there his meetings with Mrs May will be at her country home, Chequers, which offers a "more informal setting", according to a Downing Street spokesman.