Peter Navarro, a former trade adviser to President Donald J. Trump, was convicted on Thursday of two counts of criminal contempt of Congress for defying a subpoena from the House select committee investigating the Jan. 6, 2021, attack on the Capitol.

The verdict, coming after nearly four hours of deliberation in Federal District Court in Washington, made Mr. Navarro the second top adviser of Mr. Trump’s to be found guilty in connection to the committee’s inquiry. Stephen K. Bannon, a former strategist for Mr. Trump who was convicted of the same offense last summer, faces four months in prison and remains free on appeal.

Mr. Navarro, 74, stood to the side of his lawyers’ table, stroking his chin as the verdict was read aloud. Each count carries a maximum of one year in prison and a fine of up to $100,000. A hearing to determine his sentence was scheduled for January.

Speaking outside the courthouse afterward, Mr. Navarro repeatedly vowed to appeal his conviction.

“I am willing to go to prison to settle this issue, I’m willing to do that,” he said. “But I also know that the likelihood of me going to prison is relatively small because we are right on this issue.”

The jury’s decision handed a victory to the House committee, which had sought to penalize senior members of the Trump administration who refused to cooperate with one of the chief investigations into the Capitol riot.

The trial also amounted to an unusual test of congressional authority. Since the 1970s, referrals for criminal contempt of Congress have rarely resulted in the Justice Department’s bringing charges. Mr. Navarro was indicted last June on two misdemeanor counts of contempt, one for failing to appear for a deposition and another for refusing to provide documents in response to the committee’s subpoena.

The rapid pace of the trial reflected, in part, the fact that the case turned on a straightforward question, whether Mr. Navarro had willfully defied lawmakers in flouting a subpoena. Even before the trial began, Judge Amit P. Mehta, who presided over the case, dealt a blow to Mr. Navarro by ruling that he could not use in court what he has publicly cast as his principal defense: that Mr. Trump personally directed him not to cooperate and that he was protected by those claims of executive privilege.

Mr. Navarro, a Harvard-trained economist and a strident critic of China, devised some of the Trump administration’s most adversarial trade policies toward the country. Once the pandemic took hold, he helped coordinate the United States’s response by securing equipment like face masks and ventilators. But after the 2020 election, he became more focused on plans to keep Mr. Trump in power.

Mr. Navarro was of particular interest to the committee because of his frequent television appearances in which he cast doubt on the election results and peddled specious claims of voter fraud.

He also documented those assertions in a three-part report on purported election irregularities, as well as in a memoir he published after he left the White House. In the book, Mr. Navarro described a strategy he had devised with Mr. Bannon known as the Green Bay Sweep, aimed at overturning the results of the election in key swing states that had been called for Joseph R. Biden Jr.

But when the committee asked Mr. Navarro to testify last February, he repeatedly insisted that Mr. Trump had ordered him not to cooperate. By asserting executive privilege, he argued, the former president had granted him immunity from Congress’s demands.

The question of executive privilege prompted more than a year of legal wrangling over whether Mr. Navarro could invoke that at a time when Mr. Trump was no longer president. Judge Mehta ruled last week that Mr. Navarro could not raise executive privilege in his defense, saying that there was no compelling evidence that Mr. Trump had ever told him to ignore the committee.

Asked after his verdict why he had not merely asked Mr. Trump to provide testimony that corroborated his claims, Mr. Navarro said the former president was too preoccupied with his own legal troubles.

“You may have noticed that he’s fighting four different indictments in three different jurisdictions thousands of miles away, OK?” he said. “We chose not to go there.”

In closing arguments on Thursday, prosecutors and defense lawyers dueled over whether Mr. Navarro’s refusal to cooperate with the committee amounted to a willful defiance of Congress, or a simple misunderstanding.

“The defendant, Peter Navarro, made a choice,” said Elizabeth Aloi, a prosecutor. “He didn’t want to comply and produce documents, and he didn’t want to testify, so he didn’t.”

Detailing the House committee’s correspondence with Mr. Navarro, Ms. Aloi said that even after the panel asked Mr. Navarro to explain any opposition he had to giving sworn testimony, he continued to stonewall.

“The defendant chose allegiance to President Trump over compliance with the subpoena,” she said. “That is contempt. That is a crime.”

Stanley Woodward Jr., a lawyer for Mr. Navarro, countered that the government had not successfully shown that Mr. Navarro’s failure to comply was anything other than “inadvertence, accident or mistake.” Mr. Woodward presented next to no evidence in Mr. Navarro’s defense and instead sought to poke holes in the government’s case that Mr. Navarro had deliberately disregarded the committee.

“Where was Dr. Navarro on March 2, 2022?” Mr. Woodward asked, referring to the date that Mr. Navarro was instructed to appear before the panel.

“We don’t know,” he said. “Why didn’t the government present evidence to you about where Dr. Navarro was or what he was doing?”

Prosecutors also emphasized the role that Mr. Navarro’s falsehoods may have played in drawing scores of rioters to Washington to disrupt Congress’s certification of the results.

That caused Mr. Woodward to bristle, telling the jury that the government was relying on emotional descriptions to tarnish Mr. Navarro’s image, rather than proving he ever intended to blow off lawmakers.

Others in Mr. Trump’s inner circle cooperated with the panel in a more limited fashion and avoided criminal charges.

Two of Mr. Trump’s advisers, Roger J. Stone Jr. and Michael T. Flynn, appeared before the committee but declined to answer most of its questions by citing their Fifth Amendment rights against self-incrimination. Mr. Trump’s final chief of staff, Mark Meadows, and his deputy, Dan Scavino, each negotiated terms with the committee to provide documents but not testimony.

During the trial, prosecutors emphasized that Mr. Navarro could have taken a similar tack. The panel had informed Mr. Navarro that if he sought to invoke privilege, he should do so in person, as well as list any documents he believed were protected.

“Even if he believed he had an excuse, it does not matter,” Ms. Aloi told members of the jury moments before they left the courtroom to deliberate. “He had to comply with the subpoena no matter what, and assert any privileges in the way Congress set forth.”