Marino risked jail on charges of false expense claims totalling 12,000 euros ($14,000) and 6,000 euros of tax fraud.
Judge Pierluigi Balestrieri cited a "lack of proof" over the allegations as he acquitted Marino, a member of Prime Minister Matteo Renzi's centre left Democratic Party.
Marino, subject of a 600,000 euros damages claim form the municipality, had denied any wrongdoing.
"I am happy. I expected this result as I knew I was innocent and finally the truth has come out over these ignominious accusations," said Marino.
He resigned last year after two years in office and had been accused of having used a municipality credit card to pay for matters such as dinners with his wife and friends. He later repaid the sums.
However, the affair helped force him out at a time when revelations were surfacing in connection with the Mafia Capitale affair.
The affair involved a police investigation into criminal links between corrupt businessmen and politicians amid the alleged expropriation of money earmarked for city services.