Saint John was born in St. Fagondez, Spain. He held several benefices in the diocese of Burgos until his conscience forced him to resign all except one chapel. There, he said Mass daily, preached and catechized. He later studied theology in Salamanca and labored for some time as a missionary priest. He then became an Augustinian hermit.

St. John's life was marked by a singular devotion to the Holy Mass. Every night after Matins, he prayed steadfastly until offering the Eucharist with tender piety. His holiness was apparent during preaching, which effected a reformation in Salamanca. He also ended quarrels and feuds among noblemen, which were common and often fatal at the time.

St. John's boldness endangered his life. A powerful nobleman sent two assassins to kill him after being rebuked for oppressing vassals. On the night of the attack, St. John's holiness and peace of soul struck the assassins, who refused to kill him and begged for forgiveness. The nobleman fell sick; but later recovered his health and repented.

St. John met his death in defense of holy purity. A noblewoman whose companion in sin he had converted poisoned him. After months of terrible suffering borne with unwavering patience, St. John went to his eternal reward on June 11, 1479.