Little is known about this early Roman martyr, except that she refused to marry a nobleman named Flaccus, in the first century. He threatened to kill her but she died naturally after three days fasting. Petronilla is traditionally identified as the daughter of Saint Peter, though this may stem simply because of the similarity of names. It is believed she may have been a convert of the saint (and thus a 'spiritual daughter' of St Peter, or a follower or servant. It is said that Saint Peter cured her of palsy.
Roman inscriptions, however, identify her simply as a martyr. She may also have been related to St Domitilla. She was buried in the Domitilla family catacomb. During the eighth century her sarcophagus was moved to St Peter's.
Her chapel there was later used by the kings of France and by the Popes who employed Michelangelo.
Her image is often depicted in English mediaeval stained glass windows and screens. Her usual emblem is a set of keys. presumably borrowed from St Peter's. On this day, Mass is offered in the chapel for France and is attended by French residents in Rome.