This tranquil location on the eastern tip of Anglesey has remains spanning over 1,000 years. It was the site of a monastery dating back to the time of St. Seiriol, who is believed to have lived in about the 6th century. A holy well which survives may have its origins in this period. In the early 13th century the Celtic community was reorganized under the Augustinian Rule, and at this time the priory church was enlarged. This now serves as the parish church.

 The monastery at Penmon is reputed to have been founded by St Seiriol, a 6th-century holyman and friend of St Cybi. The monastery prospered and in the 10th century fine crosses were set up at its gates, but the Viking raids have destroyed all other evidence of this date. During the 12th century, revival under Gruffydd ap Cynan and Owain Gwynedd the abbey church was rebuilt, and it remains the finest and most complete example of a church of this period in Gwynedd. In the 13th century the Celtic monasteries were persuaded by Llywelyn ap Iorwerth to adopt a more regular rule, and Penmon eventually became an Augustinian priory with quite substantial conventual buildings. The priory survived the Edwardian conquest and expanded slightly, but was dissolved in 1538. The buildings passed into the hands of the Bulkeleys of Beaumaris, who enclosed much of the land as a deep park and built a fine dovecote. They also converted the prior’s lodging into a rather attractive house. Throughout this time the priory church remained in use, as it does today.

 The two high crosses are the only tangible evidence for the early medieval monastery. Both are now inside the church. One had been used as a window lintel in the later refectory; the other used to stand in the deer park but was moved to the church to prevent further weathering. Both would have probably stood close to the gates in the monastic enclosure. The smaller cross is in the south transept; the one from the deep park in the nave. Light switches to illuminate them and the nave are in the north transept. The square font at the end of the nave is decorated with three panels of very similar fret decoration, and it is possible that it was originally the base of another cross.

All three of these pieces belong to the school of sculpture which absorbed stylistic traits from northern English, Viking and Irish art. They date from the late 10th or early 11th centuries, perhaps from the relatively peaceful reign of Gruffydd ap Llywelyn, and the sculptors who created them may have had close connections with Cheshire.

The nave is quite plain, with small, high windows. The squat, conical tower is a well-known landmark. At ground level the crossing has richly carved pillars and arches. The decoration suggest perhaps skills were derived from Ireland, where the princes of Gwynedd had close contacts. The south transept is embellished with a blind arcade of chevron-decorated arches. A series of carved stones found during restoration have been reset in the south transept, where the small window contains fragments of medieval glass.

The conventual buildings date from the 13th century, when the positions of Penmon and its sister foundation on Ynys Seiriol (Puffin Island) were regularized. The cloister, now a garden, stood on the south side of the chancel. The eastern range of buildings has gone, but the southern one, containing the refectory with a dormitory above, still stands. Enter by a door on the south side which gives access to a cellar. A 12th-century gravestone which had been used as a lintel to this door stands nearby.

The Holy Well is a spring emerging from a cliff behind the church. It is reached by a path on the left just beyond the car park, which skirts the monastic fish pond. Although it is the source of water for the monastery, the structures are relatively modern. The roofed inner chamber around the pool is of brick and dates from 1710. The lower courses and lower antechamber with seats on either side may be somewhat earlier, but no medieval finds were made during recent excavations. The so-called ‘cell’ beneath the cliff on the left is of uncertain date and purpose.

Nearby, an impressive dovecot was built about 1600 by a local landowner. With its massive domed roof, it has room for nearly 1,000 birds.