The memos were critical of Trump's economic policies, claiming they could wreck the world trade system, described conflicts within the White House as "knife fights" and warned "the worst cannot be ruled out" in regard to allegations of Trump's collusion with Russia.
"We don't really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction riven; less diplomatically clumsy and inept," Darroch wrote in one of a series of leaked documents covering the period from 2017 to the present.
“The ambassador has not served the U.K. well," Trump told reporters Sunday as he left his golf resort in Bedminster, New Jersey. "I can tell you that. We’re not, we’re not big fans of that man.”
NBC News has reached out to the U.K. ambassador's office for comment.
The U.K. foreign office defended Darroch in a statement, saying that ambassadors are allowed to be candid about their views.
"The British public would expect our Ambassadors to provide Ministers with an honest, unvarnished assessment of the politics in their country," the statement said, adding an ambassador's perspective is not representative of the views of the government.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
The statement did not deny the veracity of the leak, instead condemning the apparent breach of protocol.
"It's important that our Ambassadors can offer their advice and for it remain confidential," the statement said.
"Our team in Washington have strong relations with the White House and no doubt that these will withstand such mischievous behaviour.”
Darroch, a veteran diplomat, is into his third year of what is usually a four-year posting that holds significant weight in U.S.-U.K. relations. He is viewed in Washington foreign diplomacy circles as an effective diplomat, but whether the role fits the agenda of Britain's future prime minister remains to be seen.