The President's son quoted an interview Mr Khan did with the Evening Standard last September, following an explosion in the Chelsea district of NY that left 29 people injured.
However, it is Khan's lack of response and previous statements of "terrorism being part and parcel to big city living" that has some questioning the mayor's commitment to safety and anti-terrorism policy.
Some 20 people were injured when the attacker drove a vehicle into pedestrians on Westminster Bridge and crashed into railings outside the Palace of Westminster on Wednesday afternoon.
News about Wednesday's attack in London was still filtering in at the time of this post's publication.
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Mayor of London Sadiq Khan offered his thanks to the police and emergency services, and said they had showed "tremendous bravery" during today's terrorist attack at Westminster.
"It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things", Khan told the Evening Standard.
The former Ukip leader also used his time to proclaim sometimes controversial protocols - such as Donald Trump's so-called Muslim ban - were in place to keep people safe.
Donald Trump Jr's choice to share a seven month old article, seemingly without reading it or pointing out that it was an old quote, was received with disbelief and overwhelming denigration.
"Nothing is more important to me than keeping Londoners safe", he said.