Historians used to consider, until now, that Mazarin never accepted to separate from his collections during his lifetime, and that his last will, written in 1661 a few days before his death, was the first legal formulation of his intention to found a public library. But the discovery of a document, hitherto unknown, proves that in 1650, during the Fronde, Mazarin had "given his library to the public". This decision is supported by a Royal Warrant dated June 6, 1650, which has been recently discovered. This act has just been edited and analysed in the journal Histoire et civilisation du livre. It allows to shed new light on the models, legal procedures, and the founding process of a public library in the early modern period.