The president doubled down on his criticisms of Khan during a press conference in London alongside outgoing British Prime Minister Theresa May.
"He's done a poor job, crime is up, a lot of problems," Trump reasserted on Tuesday.
The two politicians have traded jabs over Twitter throughout the duration of Trump's U.K. visit, with Trump labeling the mayor as a "stone cold loser" as he arrived in London. Khan responded by telling BBC that the president's insults reminded him of "the sort of thing an 11-year-old would do."
During an appearance on "Outnumbered", Hilton - who hosts "Next Revolution" on Fox News Channel - said Khan is simply "pandering" to his left-wing base and is considered a "failure" as mayor.
Hilton mocked Khan for characterizing his statements as a response to Trump, pointing out that Khan fired the first shot with an article over the weekend where he called Trump a "global threat."
"What a ridiculous character Sadiq Khan is. ... He's not responding. He wrote a whole article gratuitously attacking the president, so the president was responding to that," said Hilton.
Hilton, who served as a senior adviser to former U.K. Prime Minister David Cameron, said he believes President Trump has "reignited" the special relationship between the countries.
He said in his view, the relationship suffered during the presidency of Barack Obama, who "prioritized the relationship with the E.U. and Germany.
Fox News' Anna Hopkins contributed to this report.