Peter was born to a Catharist family of Verona — both his parents were Cathar perfecti — but converted to orthodox Catholicism while a student at the University of Bologna, where he heard Dominic preach. He entered the Dominican Order around 1221 and spent thirty years preaching against the Cathars across northern Italy, with such effect that the Cathar networks of Lombardy and the Veneto were broken in his lifetime.
In 1251 Pope Innocent IV appointed him Inquisitor for northern Italy. The following year, on April 6, 1252, as he travelled from Como to Milan, two assassins paid by the Cathars (Carino of Balsamo and Manfredo Clitoro of Giussano) attacked him on the wooded road at Barlassina. The first blow split his head; tradition records that as he died Peter wrote the first words of the Creed — Credo in Unum Deum — in his own blood on the ground.
He was canonized by Pope Innocent IV on March 25, 1253 — three hundred and thirty-seven days after his death. This remains the shortest interval between death and canonization in the entire history of the Catholic Church (Becket, the previous record-holder, was canonized in twenty-six months). His head reliquary at Sant'Eustorgio in Milan still bears the split he received from the assassin's axe.