But it also said Lagarde won't be fined, serve any jail time, nor receive a criminal record.
Lagarde had been accused of giving preferential treatment to businessman-turned-politician Bernard Tapie as he pursued a legal challenge against the French government.
At the time, Lagarde was French economy minister.
Tapie eventually won his case and was awarded €293 million ($306 million), plus interest. Lagarde was found to be negligent for not challenging the award of such a large sum of taxpayers' money.
She was not in court for the verdict. Her lawyer said she was in Washington.