Covering an area of some 95 km², it lies along and to the northwest and southeast of the A483 Swansea to Chester road immediately north of the town of Llandovery.

Two particularities of Llanfair-ar-y-bryn (meaning: "St Mary's on the hill") are that The area is served by trains operated between Swansea and Shrewsbury via Llandrindod Wells by Transport for Wales, which manages Cynghordy railway station, a request stop on the Heart of Wales Line from Llanelli to Craven Arms.

The community is bordered by the communities of: Myddfai; Llandovery; and Cilycwm, all being in Carmarthenshire; by Llanddewi Brefi in Ceredigion; and by: Llanwrtyd Wells; Llangamarch; Maescar; and Llywel, all in Powys.

The original Church of St Mary stands a mile outside the parish on the northeastern edge of Llandovery, on the site of the Roman auxiliary fort of Alabum.

[4] William Williams Pantycelyn (1717–1791), generally acknowledged as one of Wales's greatest hymn writers, is buried in the churchyard there, having been a resident of the parish for much of his life.