According to history, Petronilla was a virgin and martyr, from the family of Domitilla. She was Peter's "spiritual daughter" in that he baptized her and showed her the light.

Her body was removed from the catacomb of Domitilla in 750 and was translated to the imperial rotonda in the old basilica, next to the tomb of the empress Mary. The chapel with the tomb of St. Petronilla became that of the kings of France, Pepin and Charlemagne (768). Later, embellished with fine artworks it became the French National Chapel. The French ambassador, Jean Cardinal De Bilhares commissioned Michelangelo to carve the Pieta for this chapel (1499). Unfortunately, the chapel was demolished when the new basilica was erected (1606). Saint Petronilla's relics are now beneath the altar in the chapel that was consecrated by Pope Paul V in 1623.
Notwithstanding the vicissitudes of the centuries, the chapel is still the "French National Chapel". Mass on her feast day, 31 May, is attended by the French community in Rome, including the French ambassador, who venerate the "spiritual sister" of France, the "first daughter of the Church". 

Above, the altar is one of the most outstanding mosaics in the basilica, by Pietro Cristofari, first director of the Vatican School of Mosaic, after a painting by Guercino (1590-1666). In a marvelous play of chiaroscuro effects, the mosiac shows the martyrdom and apotheosis of the saint as she is received by Christ.

Saint Petronilla is also depicted in the lunette of the dome, as she is baptized by St. Peter and given Communion by St. Nicodemus.