Street, 68, had pleaded not guilty to all nine charges in an indictment that said he took advantage of his relationship with the mayor to make $30,000 a month as a consultant and minority subcontractor at the airport from 2001 to 2004, yet did little or no work.
Prosecutors also said he failed to report $2 million in consulting fees for tax purposes.
"No corruption, no fraud, no wrongdoing on the part of Milton Street. I didn't file. That's it," Street said afterward. "That's a major victory."
Prosecutors said he faces 27 to 33 months in prison, but Street's lawyer said that could be mitigated by his age, his multiple sclerosis and his years as an activist and state lawmaker.
Sentencing was set for May 22.
Younger brother John F. Street left office last month after two terms as mayor, a tenure marred by an unrelated FBI investigation of the city's pay-to-play culture. The FBI bugged John Street's office but he was never charged; his city treasurer, two bank executives and others went to prison.
On the stand, Milton Street told jurors he did nothing illegal, but believes the U.S. tax code is unconstitutional.
The jury convicted him of tax evasion for not filing tax returns for 2002, 2003 and 2004. It deadlocked on two counts that accused him of underreporting his income for 2000 and 2001.
The counts on which he was acquitted all related to the airport contracts. Co-defendant John H. Velardi Sr., 54, of Drexel Hill, who worked on the airport contract, was also acquitted of the three fraud charges he faced.
Velardi's lawyer, Guy R. Sciolla, called the airport contracting process "unsavory."
"That's business, politics in America. It's not illegal. But is it something we really want to have? Probably not," he said.
Prosecutors didn't win any felony convictions, but still argued afterward that Milton Street had tried to exploit his City Hall connections to command big money.
Street "committed a federal crime when he failed to report that income, failed to file his tax returns and failed to pay his fair share," Assistant U.S. Attorney Jennifer Arbittier Williams said.
The elder Street last year ran for an at-large seat for City Council despite the indictment, questions about his residency and the fact his nephew was also in the race.
A former hot dog vendor, he once started an amphibious duck-boat tour operation that ended up in a court battle over turf with an established tour company.
Last year, he was arrested for allegedly racking up thousands in unpaid traffic fines in New Jersey. He lives at least part-time in Moorestown.
