Later that day, Trudeau, French President Emmanuel Macron, British Prime Minister Boris Johnson, Dutch Prime Minister Mark Rutte and Britain's Princess Anne were filmed on a press pool camera talking about Trump during a reception at Buckingham Palace.
In the video, which was posted online by Canadian broadcaster CBC, Johnson can be heard asking Macron, "Is that why you were late?"
"He was late because he takes a 40-minute press conference off the top," Trudeau can be heard responding. He later confirmed his comment referred to a long and unscheduled question-and-answer session between Trump and journalists earlier in the day.
In the video, Macron's response could not be heard.
Later in the clip, Trudeau says, "You just watched his team's jaws drop to the floor." He said Wednesday that the comment referenced Trump's decision to hold the next Group of Seven meeting at Camp David, the presidential retreat.
In a tweet Wednesday, Trump announced the cancelation of a planned news conference to end the NATO summit, saying "we did so many over the past two days."
"When today's meetings are over, I will be heading back to Washington," Trump wrote.
The president dominated discussions Tuesday with news media and world leaders including Macron, Trudeau and NATO Secretary General Jens Stoltenberg, Politico reported. Trump and other world leaders spoke with reporters for about two hours, according to the news site.
Leaders of the 29 NATO states met to mark the 70th anniversary of the military alliance -- and to try to patch up differences over defense spending, the alliance’s strategic direction and member nation Turkey’s military action in northern Syria.
The two-day gathering ended with a show of unity, as the leaders declared their commitment to the alliance's principle of collective defense, saying in their final declaration that "an attack against one Ally shall be considered an attack against us all."
The Associated Press contributed to this report.