They face up to 20 years in prison and up to $250,000 in fines. A sentencing date has not been set.
The U.S. Attorney's Office argued the men used their power and influence within City Hall as leverage to force the festival's organizers into hiring union stagehands. They did it, prosecutors charged, to please Walsh, a former union official.
Walsh said he was surprised and disappointed by the conviction.
"I have made it clear from the beginning there is only one way to do things in my administration and that is the right way," the mayor said in a statement. "I have always believed their hearts were in the right place."
The mayor also said the city has taken measures to ensure employees have the tools and training to perform at the highest ethical standards. He did not specify what they were.
The case began in 2015 when the pair were charged. It was dismissed in 2018, after prosecutors failed to prove the men sought a tangible personal benefit. But earlier this year, a U.S. Appeals Court judge sent the case back for trial.
Prosecutors alleged while the music festival production company awaited permits for its event on City Hall Plaza, Brissette and Sullivan repeatedly told the company to hire members of the International Alliance of Theatrical Stage Employees, Local 11 to work the event.
The producers told them they had already entered into a contract with a non-union company and hired all of its labor, according to court documents.
But three days before the music festival was set to begin, Brissette and Sullivan insisted that half of the production company's labor force be union members, the complaint said. As a result, the production company agreed to hire nine members of Local 11 because they feared the company would be financially ruined if they did not agree to the demands, the U.S. Attorney's Office said.
The defendant's attorneys argued the prosecution failed to present evidence of threats or motive of personal gain. They said the men were doing their jobs, trying to broker a resolution to a potential labor dispute between the festival and Boston's Local 11 stagehands union.
But the 12-member jury disagreed. They deliberated for seven hours after a two-week trial.
Following the conviction, Sullivan and Brissette resigned from their city jobs. Last year, Sullivan earned $123,624, while Brissette was paid $99,909, according to city records.
"Private companies that want to do business in Boston have the right to hire anyone they want, union or not, without fear of being threatened with economic disaster by government officials," said U.S. Attorney Andrew Lelling in a statement.
Joseph R. Bonavolonta, FBI special agent, said the verdicts demonstrate that public officials cannot use their positions to extort those who choose to use non-union labor.
"The FBI will not stand by while hard-working individuals are bullied and strong-armed by public servants," he said. "Let this case be a warning to municipal workers everywhere, it is the taxpayers they serve and answer to at the end of the day."
Philip Johnston, the former chairman of the Massachusetts Democratic Party, said the convictions will not impact Walsh's political future. There have been reports the two-term mayor is considering a run for the governor's office.
"I don't think anyone believes Walsh did anything dishonest," he said. "The mayor has a reputation of honesty and integrity, and I doubt it will have any effect on him at all."