Bannon, 68, was indicted last year over his refusal to co-operate with the congressional committee probing the events leading up to the Capitol riot.
The former White House chief strategist is said to have been an unofficial adviser to Mr Trump at the time of the insurrection on 6 January 2021.
He faces up to two years in jail and up to $200,000 (£167,000) in fines.
Speaking to reporters outside court, Bannon vowed to have the case reversed on what his lawyer called a “bullet-proof appeal”.
“We may have lost the battle here today, but we’re not going to lose this war,” he said.
His sentencing has been set for 21 October.
Lawyers with the US Department of Justice had argued that Bannon felt “above the law” by ignoring a “mandatory” legal summons from the congressional committee investigating the 6 January breach of the US Capitol.
“Our government only works if people show up, it only works if people play by the rules, and it only works if people are held accountable when they do not,” prosecutor Molly Gaston said during closing statements.
“The defendant chose allegiance to Donald Trump over compliance with the law.”
Despite vowing to go “medieval” on his enemies, Bannon’s defence team rested its case on Thursday without him testifying and without calling any other witnesses.
Attorneys argued the trial against Bannon was an act of political retribution.
They asserted that, rather than ignoring the subpoenas, he believed he was negotiating on them, and also believed the deadlines in the summons were flexible, not fixed.
In closing statements, defence lawyer Evan Corcoran told the court the path his client took “turned out to be a mistake”, but “was not a crime”.
The 12-member jury panel deliberated for just under three hours on Friday before reaching its verdict.
Bannon was a key player in former President Donald Trump’s 2016 election win, serving first as his campaign chief and later taking on the role of chief strategist at the White House.
He left that position amid political fallout from a violent far-right rally in Charlottesville, Virginia, in August 2017. But the podcaster is still considered a top ally of Mr Trump.
The House of Representatives select committee investigating the Capitol riots first issued a legal summons to Bannon in September 2021.
The panel has long believed he was involved in efforts by Trump supporters to storm Congress and challenge the outcome of the 2020 presidential election.
It is particularly interested in Bannon’s communications with Mr Trump before the incident, as well as “war room” meetings held at a nearby hotel with other key figures, allegedly as part of a last-ditch attempt to thwart the certification of Joe Biden’s election win.
The day before the attack, he declared on his podcast that “all hell is going to break loose tomorrow”.
But Bannon proclaimed his innocence and defied the subpoenas, saying he would turn it into a “misdemeanour from hell” for the Biden administration.
He also maintained his conversations with the former president were covered by executive privilege, a legal principle that holds communications between presidents and their advisers to be protected from disclosure in order to allow for candid advice.
A judge, however, ruled he could not claim privilege in this case.
The January 6 committee lauded Friday’s verdict as “a victory for the rule of law and an important affirmation of the Select Committee’s work”.
Its statement continued: “Just as there must be accountability for all those responsible for the events of January 6th, anyone who obstructs our investigation into these matters should face consequences. No-one is above the law.”
