The court gave Hidalgo a suspended fine of 500 euros and ordered her to pay a symbolic euro damages to the party, as well as paying 2,000 euros costs.
The case arose from a television interview Hidalgo gave in 2012 when she was deputy to the previous mayor, fellow-Socialist Bertrand Delanoë, in which she declared that the National Front had "fought the republic [and] supported collaboration with the Nazis during the war".
The far-right party's lawyer, David Dassa Le Deist, argued that her words were not a slip-up but designed to "floor an opponent" with a "hammer-blow".
Hidalgo's lawyer, Jean-Pierre Mignard, pointed out that some of the FN's founders were "notorious collaborators" with the Nazis guilty of "unrestrained anti-Semitism" and cited former leader Jean-Marie le Pen's convictions for justifying crimes against humanity and his statement that the German occupation had not been "particularly inhuman".
The judges found that, since the FN was founded in 1972, Hidalgo's assertion was not factually correct and was expressed in a tone that was without "nuance and reserve".
"I note that for the court the Front National's honour is worth one euro," Mignard commented afterwards, adding that his client would decide whether to appeal.
"We didn't ask for any more than this," was Le Deist's reaction.
A "very serious slander" had been "justly condemned", he said.
To read Has Marine Le Pen made France's Front National respectable? with timeline history of the FN click here