The judge in the trial of Joe Fontana said the former federal cabinet minister abused the public trust when he altered a contract that he submitted for expenses in 2005.
Fontana testified the $1,700 the government paid out was not for his son's wedding.
But Judge Bruce Thomas said the former MP concocted a story for the court with the president of the club where the wedding was held that was intended to create reasonable doubt.
"While I am perplexed as to why a man of such accomplishments might choose to take these actions for the sum of $1,700, I do not find that that detracts from the strength of the Crown's case," he said.
"I'm very surprised. Shocked, devastated by obviously the verdict," Fontana said after the verdict.
He had refused to step down as mayor while his trial was under way and did not say what the outcome means for his political future, saying that he will make a statement in the near future.
He is to be sentenced July 15.