Anthony "The Mooch" Scaramucci, the former director of communications at the White House, appeared on the BBC Newsnight when he made the comments.
Speaking to host Emily Maitlis, who asked why Americans voted for Trump despite his blatant "lies," Scaramucci said: "We know he is not telling the truth.
"Probably, he even knows he is not telling the truth.
"What you need to understand about your base: they do not care."
He continued: "It is based on the seismic sensation that they had been underrepresented.
"It's the same reason why the people in your nation voted for Brexit.
"They feel like the fat cats in their society and establishment in their society have not been listening to their trials and tribulations.
"They have gone from aspiring to the working class to despair."
He added: "Everyone is lying." When do we know when a politician is lying?
"When your lips move."
Scaramucci, who recently released an explosive paint book about Trump from a very positive perspective, defended the authenticity of the President, but admitted that he did not agree with the Republican leader on many issues.
The American businessman admitted that he thought the policy of separating children "was absolutely ridiculous" and that "of course" he had expressed his opinions to Trump.
He said: "You may disagree with someone, but still, as in general, the policies you are developing.
"You can not do a black and white test: there is a lot of gray in our society."
Scaramucci's new book, entitled The Blue-Collar President, tells the "inner story" of Trump's unprecedented rise to power.
In the book, Mr. Scaramucci reveals extraordinary anecdotes from his 30-year relationship with Trump, from the first interactions in the social scene of New York City to revealing experiences in the campaign and striking details about his brief tenure as Director of Communications from the White House. .