Joseph Biggs, a onetime lieutenant in the Proud Boys, was sentenced on Thursday to 17 years in prison after his conviction on charges of seditious conspiracy for plotting with a gang of pro-Trump followers to attack the Capitol and disrupt the peaceful transfer of presidential power on Jan. 6, 2021.

Mr. Biggs’s sentence was one of the stiffest penalties issued so far in more than 1,100 criminal cases stemming from the Capitol attack and among only a handful to have been increased for fitting the legal definition of terrorism. It was just shy of the 18-year term given in May to Stewart Rhodes, the leader of another far-right group, the Oath Keepers militia, who was also found guilty of sedition.

The sentence, handed down by Judge Timothy J. Kelly in Federal District Court in Washington, kicked off a series of hearings scheduled for this week and next at which punishment will be meted out against the former chairman of the Proud Boys, Enrique Tarrio, and three other members of the group who were convicted of sedition and other serious crimes at a landmark conspiracy trial this spring. One of Mr. Biggs’s co-defendants, Zachary Rehl, is scheduled to be sentenced in front of Judge Kelly on Thursday afternoon.

The Proud Boys — who had been fighting on the streets since 2017 for a range of far-right causes — became a central focus of the F.B.I.’s investigation into Jan. 6 within days of the Capitol attack.

Aside from Mr. Biggs and his co-defendants in the sedition case — Mr. Tarrio, Mr. Rehl, Ethan Nordean and Dominic Pezzola — more than 20 other members of the group from chapters ranging from New York to Hawaii were ultimately charged in separate indictments.

The Justice Department’s prosecutions of the Proud Boys all but decapitated the group’s national leadership, which was formally disbanded after the Capitol attack, and mostly put an end to its involvement in large-scale — often violent — pro-Trump rallies in cities across the country.

But as arrests began after Jan. 6, Mr. Tarrio and his circle of lieutenants started an effort to have their followers become involved in right-wing politics in different ways. For some, that meant running for local offices or positions in county Republican organizations. For others, it meant taking part in smaller-scale protests at school boards or against L.G.B.T.Q. events.

For Mr. Biggs, the sentence effectively ended an unusual career that included a stint as a combat soldier, a job as a roving correspondent for the conspiracy theory website Infowars and a leadership role in the Proud Boys at a moment when the far-right group was thrust from the fringes of national politics and into the center of the 2020 election for their backing of President Donald J. Trump.

Mr. Biggs, one of Mr. Tarrio’s closest confidantes, helped run the Proud Boys when Mr. Trump famously called out the organization during a presidential debate against Joseph R. Biden, Jr., telling its members to “stand back and stand by.”

During tearful remarks to Judge Kelly, Mr. Biggs said that he turned to drinking — a favorite Proud Boy pastime — after coming back from combat overseas and that the only group he wanted to be affiliated with these days is “my daughter’s P.T.A.”

“I’m not a terrorist,” he said.

Judge Kelly in turn told Mr. Biggs that the attack on the Capitol, which he had helped to instigate, was a “national disgrace.”

“What happened on Jan. 6 harmed an important American custom,” Judge Kelly said. “That day broke our tradition of peacefully transferring power, which is among the most precious things we had as Americans. Notice I said ‘had’ — we don’t have it anymore.”

The Proud Boys’ sedition trial, which lasted more than three months, was one of the most significant criminal proceedings to have emerged from the Capitol attack. Prosecutors portrayed the group under the command of Mr. Biggs and Mr. Tarrio as some of the most violent members of in the enormous pro-Trump mob that stormed the building, with dozens of its members playing decisive roles in breaching barricades and assaulting the police.

In court papers filed this month, prosecutors described Mr. Biggs as “a vocal leader” of the Proud Boys and an “influential proponent of the group’s shift toward political violence,” noting that within days of Mr. Trump’s election loss he had declared that the country could face “civil war.”

Shortly after Mr. Trump posted a message on Twitter, summoning his followers to what he predicted would be a “wild” protest in Washington on Jan. 6, Mr. Biggs wrote to Mr. Tarrio, encouraging him to “get radical and get real men” to answer Mr. Trump’s call to action.

On Jan. 6 itself, Mr. Biggs took part in an episode outside the Capitol that was widely seen as a tipping point in the riot. He had a private conversation with a man in the crowd, Ryan Samsel, after which Mr. Samsel approached a barricade and confronted the police, resulting in the first breach of the Capitol’s security perimeter.

After serving eight years in the Army, some as a noncommissioned officer in combat tours of Iraq, Mr. Biggs “appreciated the tactical advantage that his force had that day, and he understood the significance of his actions against his own government,” prosecutors said.

Moreover, they noted, Mr. Biggs recorded a podcast after the riot in which he declared that the attack on the Capitol was “a warning shot to the government.”

Mr. Biggs’ contacts in the world of right-wing politics were never restricted solely to he Proud Boys. Like Mr. Tarrio, he has long had ties to Roger J. Stone, Jr., one of Mr. Trump’s political advisers. He has also been involved at the edges of far-right disinformation campaigns like the Pizzagate conspiracy theory, which falsely held that top Democrats like Hillary Clinton ran a child sex trafficking operation from the basement of a Washington pizzeria.

While working at Infowars for its proprietor Alex Jones, Mr. Biggs often covered the far-right militia movement. In 2014, for example, he followed members of the Oath Keepers to Ferguson, Mo. as the group deployed — in its own words — to protect local businesses against unrest that stemmed a failure to bring charges against a local police officer who killed a Black man, Michael Brown.

During the sedition trial, prosecutors played a brief video from Jan. 6 in which Mr. Biggs could be heard saying that he was trying to get in touch with Mr. Jones and wanted to meet up with him that day. While the two men never did seem to meet at the Capitol, Mr. Jones, who helped lead a crowd from Mr. Trump’s speech near the White House to the Capitol grounds, attracted serious scrutiny from investigators but was ultimately not charged in the inquiry.

Judge Kelly’s decision to impose what is known as a terrorism enhancement on Mr. Biggs’s sentence was one of the most consequential he made on Thursday. The measure can be applied if prosecutors can show that a defendant’s actions were undertaken in an effort to influence “the conduct of government by intimidation and coercion.”

But because Mr. Biggs did not engage in any violence against people, the terrorism enhancement emerged from a charge in which he was found guilty of damaging a government-owned fence in a way that allowed others rioters to surge forward.

While Judge Kelly said the provision was technically applicable, he was less certain — given what Mr. Biggs had actually done — that it fit in a more colloquial sense.

He noted that he had reviewed several cases involving terrorism, most concerning situations where defendants “were training to fight American troops or planning an act like blowing up a large building.” And he expressed skepticism that the Proud Boys had engaged in that kind of behavior on Jan. 6.

In response, Jason McCullough, the lead prosecutor in the case, told Judge Kelly that while the crimes committed by the Proud Boys that day did not “involve mass casualties,” they did involve an attack on Congress — one, he added that “pushed us to the edge of a constitutional crisis.”

“There’s a reason why we will hold our collective breaths as we approach further elections,” Mr. McCullough said. “We never gave it a second thought before Jan. 6 — none of us.”