The soil has been inhabited since prehistoric times: dolmens, menhirs, stone axes testify. The Romans occupied the territory also: Brick, swords and bronze tools were found. The name of Crossac (Croazac, Chroachac, Croysiac) seems to come from a Gallo-Roman Crossius melted or Crossilius.
Crossac parish was founded in the 11th century by the priory of Er which depended on the abbey of Redon.
The Lordship of Crossac called lordship, founded in the 11th century, had its headquarters at the castle of the angle. In the 18th century, it depended on the Barony of Pont-Château and its seat was Castle Bellebat. Visible locations of these two castles.
Creation of the common in 1792.
In past centuries, the farmers kneading bread at home was baked in the oven that fitted each village. Some ovens are restored again used at parties. Mills that produced flour, there remain only a few remnants of stones.