WASHINGTON — Roger J. Stone Jr., a former aide and longtime friend of President Trump, was found guilty on Friday of obstructing a congressional investigation into Russia’s interference in the 2016 election in what prosecutors said was an effort to protect Mr. Trump.

Mr. Stone, 67, was charged with lying to the House Intelligence Committee, trying to block the testimony of another potential witness and concealing reams of evidence from investigators. Prosecutors claimed he tried to thwart the committee’s work because the truth would have “looked terrible” for both the president and his campaign. He was found guilty of all seven counts he was charged with.

The government built its case over the past week with testimony from a friend of Mr. Stone and two former Trump campaign officials, buttressed by hundreds of exhibits that exposed Mr. Stone’s disdain for congressional and criminal investigators. Confronted with his lies under oath by one associate, prosecutors said, Mr. Stone wrote back: “No one cares.” They asked the jurors to deliver a verdict proving him wrong.

The evidence showed that in the months leading up to the 2016 election, Mr. Stone strove to obtain emails that Russia had stolen from Democratic computers and funneled to WikiLeaks, which released them at strategic moments timed to damage Hillary Clinton, Mr. Trump’s Democratic opponent. Mr. Stone briefed the Trump campaign about whatever he had picked up about WikiLeaks’ plans “every chance he got,” Jonathan Kravis, a lead prosecutor, said, but denied to congressional investigators that he did so.

The trial revived the saga of Russia’s efforts to bolster Mr. Trump’s chances of winning the White House at the same time that House impeachment investigators are scrutinizing Mr. Trump’s efforts to pressure Ukraine, a foreign ally, for help with his 2020 election.

Unfolding in a courtroom just blocks from the impeachment hearing room on Capitol Hill, the case resurrected a narrative that dogged Mr. Trump’s presidency until the two-year investigation by the special counsel, Robert S. Mueller III, ended last spring. Mr. Stone was accused of lying to the same House intelligence panel that is now leading the impeachment inquiry.

The jury of nine women and three men deliberated for about seven hours over two days before convicting Mr. Stone, a 40-year friend of Mr. Trump and well-known political provocateur. Mr. Stone listened impassively to the verdict, eyebrows arched and one hand in his pocket. He and his lawyers, still under a gag order imposed by the judge months ago, left the courthouse without comment.

Within minutes of the verdict, Mr. Trump protested on Twitter that it was unfair. “So they now convict Roger Stone of lying and want to jail him for many years to come,” Mr. Trump wrote, though his own administration’s Justice Department waged the prosecution.

“Didn’t they lie?,” he added before naming nearly a dozen favorite targets of his ire and suggesting they had lied without punishment, including Hillary Clinton, Mr. Mueller, the former F.B.I. director James B. Comey and Representative Adam B. Schiff, who heads the House Intelligence Committee.

Testimony by Rick Gates, Mr. Trump’s deputy campaign chairman, called into question Mr. Trump’s answers to queries from Mr. Mueller. Mr. Trump, who agreed to respond to questions only in writing rather than sit for an interview, said he could not recall the specifics of any of 21 conversations he had with Mr. Stone in the six months before the election. Mr. Stone told House investigators that he never discussed his conversations with an intermediary to WikiLeaks with anyone involved in the Trump campaign.

But in one of the trial’s most revealing moments, Mr. Gates recounted a July 31, 2016, phone call between Mr. Stone and Mr. Trump, just days after WikiLeaks had released a trove of emails embarrassing the Clinton campaign. As soon as he hung up with Mr. Stone, Mr. Gates testified, Mr. Trump declared that “more information” was coming, an apparent reference to future releases from WikiLeaks that would rattle his political rival.

Mr. Stone, 67, joins a notable list of former Trump aides convicted of lying to federal authorities. It includes Mr. Gates; Michael T. Flynn, the former national security adviser; Michael D. Cohen, the president’s longtime personal lawyer and fixer, and George Papadopoulos, a former Trump campaign aide. And his former campaign chairman Paul Manafort, who was also once Mr. Stone’s partner in a political consulting firm, was convicted of a string of financial crimes and is serving a seven-and-a-half-year prison term.

The most serious charge against Mr. Stone, witness tampering, carries a maximum penalty of 20 years in prison. The other charges carry a maximum of five years each. But the punishment for a first-time offender like Mr. Stone will almost certainly be much lighter.

Working against Mr. Stone could be his multiple run-ins earlier this year with Judge Amy Berman Jackson, who is overseeing the case and will preside over sentencing, set for Feb. 6. After a series of infractions, including posting a photo of the judge with an image of cross-hairs next to her head on Instagram in February, she banned him from social media.

After the verdict was announced, prosecutors asked Judge Jackson to order Mr. Stone into custody, saying he had violated her gag order by making comments the previous day in a broadcast by Infowars, a far-right website run by Alex Jones. But the judge rejected their motion, saying that in recent months Mr. Stone had complied with her orders and proof of a new infraction was not entirely clear.

In a video posted to Infowars, titled “Roger’s Emergency Message To America,” Mr. Jones said that Mr. Stone had told him that he expected to be convicted and wanted Mr. Trump to pardon him. “I appeal to the president to pardon me, because to do so would be an action that would show these corrupt courts that they’re not going to get away with persecuting people for their free speech or for the crime of getting the president elected,” he said Mr. Stone told him.

Mr. Stone’s lawyers argued that the prosecution’s case was based on speculation and false assumptions about Mr. Stone’s motives. They pointed out that Mr. Gates had no knowledge about what was said during the phone call between Mr. Stone and Mr. Trump. Bruce S. Rogow, the lead defense lawyer, told jurors that Mr. Stone had no reason to lie in order to protect the president nearly a year after Mr. Trump had won the election, and that Mr. Stone had simply confined his answers to the strict parameters of the committee’s inquiry.

Besides Mr. Gates, the trial featured testimony from another well-known former Trump aide, Stephen K. Bannon, who led Mr. Trump’s campaign through its final three months and served as a top White House adviser early in the administration. He and Mr. Gates both testified that Mr. Stone portrayed himself as the campaign’s link to WikiLeaks, even though he and his lawyers now assert that was mere braggadocio.

Much of the trial revolved around interactions between Mr. Stone and Randy Credico, a New York radio host and comedian who Mr. Stone identified to congressional investigators as his intermediary with Julian Assange, the founder of WikiLeaks. Prosecutors said that Mr. Stone pressured Mr. Credico not to cooperate with the House committee because his account would have exposed Mr. Stone’s lies.

During their tortuous 17-year friendship, Mr. Credico said, Mr. Stone repeatedly played him as a “patsy,” including publicly blaming him for his own misdeeds. He said he misidentified him to the committee as his go-between with WikiLeaks in the summer of 2016 despite his repeated pleas to Mr. Stone to tell the truth.

In fact, prosecutors said, in late July of that year, Mr. Stone had dispatched another associate, an author and conspiracy theorist named Jerome Corsi, to “get to Assange.” In one of the mysteries of the trial, prosecutors never called Mr. Corsi to testify. Asked why, Mr. Corsi replied in a text message: “Ask them — I don’t know.”

Text messages and other evidence suggested that Mr. Stone alternately flattered and threatened Mr. Credico in an effort to ward off his testimony. At one point, he pretended that he had written a letter to the House committee characterizing Mr. Credico as highly talented and successful.

He repeatedly urged Mr. Credico to “Do a Frank Pentangeli,” referring to a character in the movie “The Godfather: Part II” who gave false testimony during a Senate hearing on organized crime. Borrowing a quote from Richard Nixon to a top aide during the Watergate cover-up, Mr. Stone texted Mr. Credico in late 2017: “Stonewall it. Plead the fifth. Anything to save the plan.”

If he refused to go along, Mr. Credico testified, Mr. Stone threatened to retaliate against him and Margaret Ratner Kunstler, a lawyer for Mr. Assange and one of Mr. Credico’s dearest friends. Prosecutors described Ms. Kunstler as a particularly effective “pressure point” with Mr. Credico, an unmarried man with no children and a 34-year history of alcohol abuse.

Mr. Stone “knew that when the time came he would be able to bend Randy Credico until he broke,” Mr. Kravis, the prosecutor, told the jurors in his closing arguments. Mr. Credico ultimately asserted his Fifth Amendment rights against self-incrimination and refused to testify to the House committee.

Prosecutors argued that jurors had black-and-white proof that Mr. Stone had lied to the House committee when he said he had no electronic communications with Mr. Credico, describing him as “not an email guy.” In fact, they exchanged more than 1,500 emails and text messages between June 2016 and September 2017, including 72 text messages on the day of Mr. Stone’s testimony.

Because Mr. Stone misled them, prosecutors said, lawmakers failed to pursue promising leads and arrived at inaccurate conclusions in their final report on Russia’s election interference. For instance, they said, the committee never discerned the full scope of contacts between Mr. Stone and the Trump campaign about WikiLeaks.