Though his parentage was long unknown, it is now established that he was the son of Edward Peyto of Chesterton, Warwickshire, and Goditha, daughter of Sir Thomas Throckmorton of Coughton.

Entering the Observant branch of the Franciscan Order, he became known for his holiness of life, and was appointed confessor to Henry VIII's daughter Mary.

On Henry VIII's death in 1547, Petow's reputation was greatly enhanced, as reported by Gilbert Burnet in his History of the Reformation of the Church of England, when Henry's coffin, having sustained some damage from jolting along the rough roads to Windsor, was placed at the former Sion Abbey for a night, where some bodily fluids mixed with blood leaked through a cleft in the lead coffin onto the pavement; the next morning, when a workman came to repair the damage, a dog crept up and was observed licking up the fluid, in apparent fulfillment of Petow's prophecy.

[3] Nevertheless, Petow did not claim the bishopric even on the accession of the Roman Catholic Mary I in 1553, but resigned the see and retired to his old convent at Greenwich.

There he remained till Pope Paul IV, who had known him in Rome and highly esteemed him, decided to create him cardinal and papal legate in place of Pole.