Trump accused Khan -- London's first Muslim mayor -- of offering a "pathetic excuse" for comments that the US president earlier misconstrued about policing in response to Saturday's attack, which left seven people dead.
Khan had told Londoners there was "no reason to be alarmed" about an increased police presence in the coming days.
Trump has leapt on the London attacks as evidence of the need for hardline security measures in the United States, including a ban on travellers from some Muslim countries.
His comments caused outrage among British officials.
"I think Sadiq Khan is doing a good job and it's wrong to say anything else -- he's doing a good job," she told a press conference on Monday.
Meanwhile, crowds gathered by London's beloved Tower Bridge on Monday -- many Muslims among them -- to grieve for the victims of the city's second jihadist attack in three months.
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