"He said I should sue the European Union", she said with a wry smile.
However, Mrs May continued to insist a deal was possible during a press conference with the President the next day - when he suggested she could take a different approach to negotiations with the EU. Don't walk away from the negotiations.
"But it was strong advice".
Though Mr Trump later contradicted his comments by then promising a great USA trade deal, the president made clear his admiration for the 54-year-old Mr Johnson, who Trump said would one day make a great British prime minister.
That meant MPs heading back to their constituencies to gauge the strength of sentiment over Theresa May's Brexit plan.
Arriving straight from a North Atlantic Treaty Organisation summit that was predictably contentious, the USA president has been greeted with massive protests on his first trip to London since taking office. "If we don't, we risk ending up with no Brexit at all", she wrote in an article in the Mail on Sunday newspaper.
He also said she would have to change her policy in order to get it through Parliament, without having to rely on Labour votes. He said he had told May how to conduct Brexit negotiations, "but she didn't listen to me".
Not so she says, it had been "in gestation for some time".
The comments come after a contentious visit from the US president, during which he criticized May's approach to Brexit.
"The weather is handsome and this place is incredible".
President Trump said: "Well, I fully intend to". He waved as they heckled him from a hill overlooking the Ailsa championship course of his golf resort.
In a very humanizing moment, Trump admitted, "I was walking up and I was saying (to Melania): "Can you imagine my mother seeing this scene?" While much has been made of the president's domineering father, Fred Trump, less is known about his mother. Environmental group Greenpeace have claimed responsibility for the stunt.
He told Today: "I'm afraid it is neither beloved by Remainers or Leavers".
"It's called fake news", Mr Trump declared.
Trump said that one of them, former foreign secretary Boris Johnson, "would be a great Prime Minister".
"So let's just keep our eyes on the prize here. Great Britain? You understand what I'm saying?"
She added: 'I have got to be hard-headed and practical about this and do it in a way that ensures we get the best interests for the United Kingdom'.
"Some of them are protesting in my favor, you know that?"