 Fryderyk Józef Moszyński was a son of Jan Kanty Moszyński, Grand Treasurer of the Crown ("podskarbi wielki koronny"), and Fryderyka Augusta, an illegitimate daughter of Augustus II the Strong, king of Poland, and his mistress Countess of Cosel.He was posthumus (his father died on 15 September 1737).He had an older brother, August Fryderyk Moszyński.