Stewart Rhodes, founder of the far-right Oath Keepers militia, was found guilty of sedition for his role in the 6 January, 2021 attack on the US Capitol by supporters of former president Donald Trump.
Another member of the Oath Keepers, Kelly Meggs, was also found guilty of seditious conspiracy while three co-defendants were acquitted of the same charge, which carries a sentence of up to 20 years in prison.
The 57-year-old Rhodes, an eyepatch-wearing former soldier and Yale law school graduate, and the four other members of the group were accused of plotting an armed rebellion to overturn the results of the November 2020 presidential election won by Democrat Joe Biden.
The verdict capped a nearly two-month, high-stakes trial.
Hundreds of Mr Trump's supporters have been arrested for their roles in the assault on Congress but they have faced lesser charges than those lodged against Rhodes and the other four Oath Keepers.
While Rhodes and Meggs were convicted of the rarely pursued charge of sedition, all five defendants were found guilty of obstructing an official proceeding and other lesser charges.
During the trial, the Justice Department said Rhodes and the Oath Keepers "concocted a plan for an armed rebellion... plotting to oppose by force the government of the United States."
Prosecutors showed videos of the attack by dozens of group members dressed in military-style combat gear.
The defendants characterised the case as a political trial carried out by Mr Biden's administration against supporters of his predecessor, Mr Trump.
The 12-person jury deliberated for nearly three full days before reaching a verdict in the closely watched case.
A not-guilty verdict on the sedition charges for all five defendants would have been a setback for the Department of Justice, which plans to try members of the Proud Boys, another right-wing extremist group, on the same charges.
During the trial, prosecutors accused the Oath Keepers of stocking weapons at a hotel near Washington and joining the crowd that stormed the Capitol in a bid to block the certification by Congress of Biden's election victory.
Rhodes did not personally enter the building but directed his followers like a battlefield general, prosecutors said.
Rhodes took the witness stand during the trial and denied his group planned to assault the Capitol, saying they were in Washington only to provide security at rallies.
"It was not part of our mission for that day to enter the Capitol for any reason," Rhodes said.
Speaking in military terminology, he admitted that a number of Oath Keepers went "off-mission" and entered the building.
He said Meggs, the head of the large Florida chapter of the Oath Keepers, was "an idiot" for taking his people inside.
"I think it was stupid to go into the Capitol. It opened the door for the political persecution of us. And that's where we are," Rhodes told the court.
Prosecutors showed the jury text messages between Rhodes and his followers that called for action if Mr Trump himself failed to act to prevent certification of Biden as the next president.
US Attorney General Merrick Garland named a special counsel earlier this month to oversee the investigation into Trump's efforts to overturn the election result, and his role in the Capitol assault.