He promoted a proposed travel ban on visitors from six mostly Muslim countries and refugees from around the world that has been blocked by US courts.
It's unclear whether the president has conveyed his requests to the Justice Department, which he oversees, in a forum other than Twitter.
Trump removed this context, however, giving off the assumption that Khan had suggested Saturday night's attack, which left seven people dead and many more injured, was not worth being alarmed about.
ISIS has claimed responsibility for the attack.
"Part and parcel of living in a great global city is you have to be prepared for these sorts of things, you have to be vigilant, you have to support the police doing an incredibly hard job, you have to support the security services", Khan said.
London police chief Cressida Dick said that, while some of the recent attacks in Britain had worldwide dimensions, they had a largely domestic "centre of gravity".
He said Sunday that there was "no reason to be alarmed" at the sight of a massive armed police presence in the city's center - a notion that sent the president into overdrive.
Later, the mayor's spokesman said he was too busy to respond to Trump's "ill-informed" tweet. At least 12 people were arrested Sunday, including five men and seven women ranging in age from 19 to 60.
"We need to be smart, vigilant and tough", Trump tweeted out. Two additional addresses were searched on Monday.
A Facebook post by a family friend shared hundreds of times on Monday appealed for information about the 21-year-old, saying she usually rang her mother daily. She used a speech at the start of her election campaign to accuse European Commission President Jean-Claude Juncker - an important figure in the coming Brexit talks - of trying to sabotage the vote.
Trump said he had spoken with May to express America's "unwavering support" and offer us assistance as the British government works to protect its citizens and bring the guilty to justice.
The company, which has had a rough stretch that has included accusations of deceiving local authorities, stealing another company's technology, and having a toxic workplace culture, initially eliminated the surge but then decided all rides taken in Central London during the attack were to made free and refunded all affected passengers.
Trump's statement contradicted his administration's stance that his directive was a not a travel ban. The first order, which was signed at the end of his first week in office, was hastily unveiled without significant input from top Trump national security advisers or the agencies tasked with implementing the order. The Romanian government recently announced it will spend tens of millions of dollars on new weaponry, which, given Trump's speech at the North Atlantic Treaty Organisation meeting in Brussels, should be music to the USA president's impatient ear.
Iraq was removed from the list of banned countries in the second order and an indefinite halt to entry from Syrian refugees was replaced by a temporary pause. The high court also is being asked to uphold the constitutionality of the Trump travel policy, which lower courts have blocked because it shows anti-Muslim prejudice.