The incident occurred after Maurice Boisart spotted the youth climbing over a fence in the northern town of Cousolre in August 2010.
After being reprimanded, the boy allegedly insulted him and made a death threat, leading the mayor to deliver his slap.
"I know it's reprehensible, but I don't deserve to be punished for it," said Boisart. "It wanted to teach him a lesson."
The mayor refused to plead guilty in an earlier hearing, preferring to take the matter to a public trial so he could explain himself.
Le Figaro newspaper reported that the mayor said the incident had taken a terrible toll on him.
"In just a few days my hair turned white to the point that people thought I was ill," he said.
He said the affair was "symbolic of the problems mayors face when dealing with numerous incivilities."
The local prosecutor asked for the mayor to be forced to pay a fine of €500 ($380) while the plaintiffs asked for €1000 to cover their legal costs.
The mayor must now await judgement from the court to know whether he will be forced to pay.