Stewart Rhodes, leader of the Oath Keepers, was convicted of seditious conspiracy by a US jury on Tuesday for a plan to stop the peaceful transfer of power after Donald Trump lost the 2020 US presidential election to Joe Biden.
The sentence carries up to 20 years in prison.
Tuesday's verdict was a victory for the Justice Department, which has maintained that the January 6 attack on the Capitol was a violent "assault" on US democracy.
"Today the jury returned a verdict convicting all defendants of criminal conduct, including two Oath Keepers for seditious conspiracy against the United States," US Attorney General Merrick Garland said in a a statement.
"The Justice Department is committed to holding accountable those criminally responsible for the assault on our democracy on January 6, 2021."
Rhodes founded the Oath Keepers, a right-wing militia group, in 2009.
Prosecutors said he stood outside the Capitol building on the day of the insurrection and acted as a general while the group's members stormed the building.
A leaked list in September showed that hundreds of elected officials, military personnel and law enforcement officers were on the militia group's membership list.
Kelly Meggs, who led the Oath Keepers inside the Capitol, was also found guilty of seditious conspiracy.
Co-defendants Kenneth Harrelson, Jessica Watkins and Thomas Caldwell were acquitted.
The Oath Keepers members were the first of about 800 people accused in the riot to go on trial.
The Justice Department said Rhodes and his co-defendants “concocted a plan for an armed rebellion … plotting to oppose by force the government of the United States”.
They were also accused of creating a “quick-reaction force” that prosecutors said was positioned at a nearby Virginia hotel and was equipped with weapons that could be sent to Washington if needed.
“As this case shows, breaking the law in an attempt to undermine the functioning of American democracy will not be tolerated,” FBI Director Christopher Wray said in a statement.
Fifty witnesses testified during the trial. Rhodes and two of his co-defendants testified in their own defence.
They denied planning an attack or trying to block Congress from certifying the 2020 election results.
The defendants said the trial was brought on by President Joe Biden's administration to punish supporters of Mr Trump.
Agencies contributed to this report