The attack led to members of the group storming the building after Joe Biden was officially confirmed as the next president.
Tarrio, along with three other members of the group, were found guilty of seditious conspiracy – which is conspiring against the legitimacy of the state.
During the election campaign, Donald Trump spoke directly to the Proud Boys, known as a militant, gun-carrying group, telling them to “stand back and stand by”.
The charges carry up to 20 years in prison.
In a big milestone, it means the US Justice Department has now secured seditious conspiracy convictions against leaders of two right-wing groups, the other being the Oath Keepers, who were said to be doing everything they could to keep Joe Biden out of the White House.
Tarrio was not in Washington DC on the day of the attack – he had been arrested two days before and ordered to leave the city.
However, prosecutors said he led and orchestrated proceedings carried out by his group, which saw itself as “Trump’s Army”, on 6 January.
The Proud Boys were “lined up behind Donald Trump and willing to commit violence on his behalf”, prosecutor Conor Mulroe said in his closing arguments during the trial.
Prosecutors used hundreds of messages between members of the group in evidence that showed the Proud Boys peddling conspiracy theories and misinformation about the notion of a “stolen election”, and what might happen when Joe Biden took office.