Atto was a Frankish monk and theologian who became Bishop of Vercelli (924/5–960/61, as Atto II), then in the Kingdom of Italy. Atto was the son of a certain viscount Aldegarius. In 933 he became Grand Chancellor to the young king of Italy, Lothair II, receiving in return several donations and privileges for his see.
During his episcopate he garnered fame throughout Europe by his zeal for the spiritual and temporal welfare of his diocese and of the church in general. He wrote a short treatise on ethics, entitled Polypticum or Perpendiculum, which was first published alongside eighteen of his surviving sermons by Angelo Mai in 1832. He also compiled canon law, including some of the False Decretals, in his Canones, which also contains provisions of his own invention, which are of value for the study of contemporary ecclesiastical life and manners in northern Italy. The first editions of several of his writings were published by Luc d'Achery (1609–85) in the eighth-volume Spicilegium.