Former Trump aide Peter Navarro has been convicted of contempt of Congress for refusing to co-operate with an inquiry into alleged efforts to overturn the 2020 election result.
Prosecutors said Navarro acted "above the law" by ignoring a subpoena from a congressional investigation.
He faces up to a year in prison for each of the two contempt counts.
Another key Trump ally, former strategist Steve Bannon, was convicted last year of contempt of Congress.
Outside the court in Washington DC on Thursday, Navarro said it was a "sad day for America", vowing to appeal all the way to the Supreme Court.
"This is the first time in the history of our republic," he said, "that a senior White House adviser, an alter ego of the president, has ever been charged with this alleged crime."
He argued that the Department of Justice has had a policy for more than 50 years that senior White House advisers could not be compelled to testify before Congress.
"Yet they brought the case," Navarro said.
He was found guilty by the 12-member jury after four hours of deliberations, following a trial that lasted two days.
As well as an appeal, Navarro's lawyers are motioning for a mistrial, alleging that jurors went outside court during their deliberations and encountered protesters.
Navarro, who served as senior trade adviser to former President Donald Trump, was served with a subpoena by a US House of Representatives select committee in February 2022.
But he did not hand over any of the requested emails or documents or appear to testify before the Democratic-led panel.
The committee had hoped to question Navarro about efforts to delay certification of the 2020 election, according to a former staff director for the panel who testified in court.
In his 2021 book, In Trump Time, Navarro said he was the architect of a strategy to challenge the election results, claiming widespread voter fraud.
The plan was for congressional Republicans to delay certification of President Joe Biden's victory.
Navarro called this strategy the Green Bay Sweep, a reference to a tactic in American football.
The House committee said Navarro's claims of massive ballot fraud had been exposed as baseless by state and local officials.
Navarro was indicted in June last year and arrested by FBI agents at a Washington airport as he was boarding a flight to Nashville, Tennessee.
During their closing arguments, prosecutors said Navarro chose his allegiance to Mr Trump over complying with the subpoena.
"That is contempt. That is a crime," prosecutor Elizabeth Aloi told the court.
Navarro's lawyer, Stanley Woodward, presented little evidence during the trial and instead sought to discredit the prosecutor's case.
When contacted by the committee, Navarro said Mr Trump had instructed him to cite executive privilege.
This is a legal principle that allows certain White House communications to be kept under wraps.
But last week, Judge Amit Mehta, an Obama nominee, ruled there was no evidence that Mr Trump or executive privilege could have permitted Navarro to ignore the committee's summons.
In addition to a maximum sentence of a year in prison for each count, Navarro also faces fines of up to $100,000 (£80,000).
His sentencing is scheduled for January.
Bryan Lanza, a former Trump campaign adviser, told the BBC the prosecution seemed politically motivated.
"It is not uncommon for Congress to hold former or serving members of presidential administrations in contempt," he said.
"It is uncommon for the actual justice department to go forward with these prosecutions."
He cited the example of the former US Attorney General Eric Holder, under Democratic President Barack Obama, who was found in contempt of a Republican-controlled Congress in 2012 for refusing to hand over subpoenaed documents, but was not criminally prosecuted.
"We're going down a dangerous route by escalating these things," said Mr Lanza.
"That's not good for our system of government," he added.
Former Trump campaign strategist Steve Bannon was convicted of two counts of contempt for defying the House committee's legal summons in July 2022.
Bannon was sentenced to four months in jail, but has remained free while his defence team appeals the conviction.