Petroc was a Welshman of noble birth who was educated an a monastery in Ireland. He set out to spread Christianity and his boat was carried by the wind and tides to Padstow, where the Saints Wethinoc and Samson were established. Petroc and the other saints built a Celtic monastery on the headland above the estuary, complete with school, hospital, library and cells for the monks. Saint Petroc travelled from this base throughout Cornwall, and to Wales and Brittany. He died at nearby Treravel when returning from one of his journeys. Petroc built the first church on the present site in Padstow in the 6th century, but it was destroyed by Vikings in 981. The only items remaining from this period are the head of a Celtic cross outside the south door, which has been mounted on a modern base, and the base of a very large cross near the south-east gate. The church was rebuilt in the 12th century, and the present-day church dates from the period 1425-1450.

Petroc's remains were stolen in the 12th century and were taken to the Abbey of Meen in France. A deputation visited the Abbey and successfully negotiated for their return, since when they have been held in a painted ivory casket in St. Petroc's Church in Bodmin.