But as is so often the case with the US president, pesky facts demonstrate he is once again a little wide of the mark.
In a series of tweets on Saturday night, the president said Khan is “destroying the City of London” after three people were killed in two stabbings and a shooting.
But perhaps the president should be directing his ire closer to home – crime stats lay bare just how much safer London is than a number of US cities, none of which have been on the receiving end of a presidential Twitter storm.
In 2018, the UK capital saw 1.8 murders per 100,000 people, fewer than New York, Austin and San Diego.
And when compared to Baltimore, New Orleans and Detroit, the picture is even starker.
The murder rate in Baltimore is a whopping 31 times higher per capita. New Orleans is 22.4 times higher and Detroit 22 times.
The US president and mayor of London’s history of trading blows can be traced back to 2015, when Khan reacted to Trump’s plan for a “Muslim ban”.
In December 2015, he said in a BBC interview he hoped Trump would lose the presidential election “badly”.
Roughly five months later, when Khan was elected mayor in May 2016, Trump laid on some unusual praise: “If he does a great job, frankly that would be a terrific thing.”
In comments reported by the New York Times, the future president also hinted Khan, a Muslim, would be “exempt” from the proposed ban, the Press Association reports.
But days later the relationship soured after Trump admitted he was “offended” by Khan’s comment that his views of Islam were “ignorant”.
“When he won I wished him well,” Trump told Good Morning Britain. “Now, I don’t care about him. It doesn’t make any difference to me about him.”
The feud intensified in the aftermath of the London Bridge attacks in June 2017, when a controversial tweet by Trump opened him up to widespread criticism.
“At least 7 dead and 48 wounded in terror attack and Mayor of London says there is “no reason to be alarmed!””, the president wrote.
Khan’s spokesman released a scathing rebuke, pointing out the comment was taken out of context.
The mayor had “more important things to do than respond to Donald Trump’s ill-informed tweet”, he said.