Welsh saints often come in family clusters, and the Deiniols are one of them. Saint Deiniol the elder - sometimes called Deiniol Wyn, 'Deiniol the Blessed' - founded the monastery of Bangor on the mainland in the 6th century and became the first Bishop of Bangor. His son Deiniol Fab - 'Deiniol the Younger' - is said by 19th-century writers to have founded the church at Llanddaniel Fab around 616, giving the village its present name (Llan- meaning 'church of'). No documentary evidence survives from anywhere near that period. What we have is a strong local tradition and a place-name that consistently points back to the saint. No part of the original 7th-century building survives. A later structure - perhaps 16th-century, perhaps earlier - was itself replaced in the 19th century by the church that stands today.

When the Victorians knocked down the old church to build a new one, they kept some of the older fabric. The vestry's north door has medieval doorjambs, and the keystone of its pointed arch - also medieval - is a carved human face, weathered but recognisable, looking out at whoever opens the door. Some sections of the nave walls may be from the previous building too, according to the 1937 survey by the Royal Commission on Ancient and Historical Monuments in Wales. The octagonal granite font is older than the present church. The porch contains an 18th-century memorial moved from the predecessor. These small fragments are how a village church preserves its history when the building itself is reset every few centuries.

The parish has been served by two clergymen whose names crop up in Welsh cultural history. Henry Rowlands, priest at Llanddaniel Fab and nearby parishes from 1696 onward, wrote Mona Antiqua Restaurata - 'Mona Restored', a history of Anglesey - published in 1723. It was one of the first serious antiquarian accounts of the island, and Rowlands has been alternately praised for its pioneering ambition and criticised for its enthusiasm for Druidic explanations of every prehistoric monument he encountered. Isaac Jones, curate here from 1840 until his death in 1850, was a translator. Both men remind you that small rural parish posts in Wales were often held by men with serious scholarly lives running alongside their pastoral work. The 19th-century antiquarians Angharad Llwyd (in 1833) and Harry Longueville Jones (in 1846) both wrote about the old church before its Victorian replacement. Llwyd called it 'a very ancient and dilapidated structure'. Jones called it 'so much altered by successive reparations, that little of its original architectural character has been preserved'.

The current building is in Early English style - Gothic Revival, built from rubble masonry dressed with limestone, slate-roofed. The plan is simple: nave with no aisle, chancel at the east end, porch on the south side, vestry on the north. The west end terminates in a bellcote with a single bell and a cross. Three lancet windows light the east end of the chancel; two-light windows run along the nave. Inside, the walls have panelling at the bottom with painted plasterwork above, exposed roof timbers, pews and altar of pine, the granite font with carving on each face. A 1937 inventory recorded a plain silver communion cup dated 1796-7. By a 2006 inspection there were cobwebs everywhere but the pews and the organ were still in place. The building is Grade II listed, since 30 January 1968, recognised by Cadw as 'coherently designed in an early Gothic style which is apt for its scale' and 'a good example of a simple 19th-century rural church'. The village's active worship has moved to nearby Llanfairpwll. What remains here is the building, the door with the carved human face, and the long, quiet lineage of the place.