"It is nice, and I am sure she will do excellently".
He warned British ministers they must be careful not to jeopardise intelligence-sharing by letting Chinese firm Huawei Technologies Co Ltd into Britain's new 5G mobile phone network.
The US president went on to express confidence that he would receive a warm welcome in Britain, despite the protests that are expected to accompany his visit.
Boris Johnson and US President Donald Trump.
As the United Kingdom braces for Mr Trump's visit with a multi-million-pound security operation, Mr Trump said he had been paying close attention to the Tory leadership contest, which will decide the UK's next prime minister. He resigned as foreign secretary a year ago in protest at May's Brexit strategy.
Comments like that appear to have won the president's support.
"Yup, I like him", he said. "I think they've done a good job".
"He has always been very positive about me and our country".
He also didn't think that Johnson's extra-marital affairs - and his lovechild - would necessarily hinder his chances. "I think today it matters much less".
In the interview, Mr Trump also repeated his criticism of how Theresa May has handled Brexit negotiations, saying Britain had allowed the European Union to "have all the cards".
Trump said: "I actually have studied it very hard". "They had nothing to lose. They didn't give the European Union anything to lose", he said.
Ball's lawyers presented papers to the court claiming Johnson had deliberately misled the public during the Brexit referendum campaign in 2016 and then repeated the statement during the 2017 snap general election called by Theresa May.
He said: "I'm trying to make it so that Iran does not have nuclear weapons".