Marino said in a letter addressed to Romans that his resignation was not an admission of guilt and he cited Italian law that says he could rescind it within 20 days.
He insisted he was being made the fall guy for having rooted out corruption and mafia infiltration in city hall from the previous administration, and vowed to expose the truth.
Rome prosecutors this week opened an investigation into Marino's use of his city hall credit card after questions arose about whether he expensed family dinners, claiming they were official business.
It wasn't enough.
His Democratic Party, which had been lukewarm to his administration for months anyway, finally yanked its support and made clear that he had lost his mandate.