Mills and Berlusconi have both denied the accusations.
The British lawyer maintained his innocence in a statement issued after the court handed down its sentence.
“I am naturally very disappointed by this verdict. I am innocent, but this is a highly political case,” he said.
“The judges have not yet given their reasons for their decision, so I cannot say how they dealt with the prosecutor’s own admission that he had no proof.
“I am hopeful that the verdict and sentence will be set aside on appeal.”
Mills, who specialises in tax havens, is the estranged husband of Tessa Jowell, Britain’s Olympics minister.
Speaking in London on Tuesday, Jowell said the verdict and sentence were a “terrible blow” to Mills.
“Although we are separated, I have never doubted his innocence,” she said.