Trump commented on the vehicle and knife attack that killed at least seven people in London at the conclusion of a Sunday night fundraiser for Ford's Theater, scene of one of the most famous acts of bloodshed in American history: the assassination of President Abraham Lincoln.
U.K. Prime Minister Theresa May on Sunday said the uncoordinated attacks months apart were evidence of a new form of copycat attacks.
Both men lived in the east London neighborhood of Barking.
The Islamic State militant group, which is losing territory in Syria and Iraq to an offensive backed by a USA -led coalition, claimed responsibility for the London Bridge attack, though it is unclear whether the attackers had links to the group.
"We believe we are experiencing a new trend in the threat we face, as terrorism breeds terrorism", she said.
Naming Butt and an accomplice, police admitted he was known to them and MI5, but that "there was no intelligence to suggest that this attack was being planned and the investigation had been prioritised accordingly".
While most of the country's political parties have chose to suspend campaigning temporarily, the upcoming general elections scheduled for Thursday have not been cancelled. A parliamentary election takes place on Thursday and May's spokeswoman said the government was working closely with police on security for the vote.
A YouGov poll of 1000 Londoners published on Monday found Mr Khan was more trusted than either Mrs May or national Labour leader Jeremy Corbyn to make the right decisions about keeping Britain safe from terrorism. Some residents cooped up inside all day Sunday emerged from their homes for the first time since the attacks.
Trump said London Mayor Sadiq Khan had offered a "pathetic excuse" and "had to think fast on his "no reason to be alarmed" statement". He said many people were "very anxious that she was at the Home Office for all this time, presided over these cuts in police numbers, and now is saying that we have a problem". 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.
Dick said Monday the attack was "ghastly", but that Londoners are pulling together and refusing to be cowed by extremists.
"Try to imagine the United Kingdom prime minister attacking the Mayor of NYC the day after 9/11", said European Parliament cabinet member Simon O'Connor.