Altarnun is a village in the E of the county, 7 miles W of Launceston on the NE edge of Bodmin Moor. The church is at the N end of the village centre, and is one of the largest in the county, with a W tower second only to Probus in height. This is of 3 storeys, begun in the late-14thc and completed in the mid-15thc. and built of ashlar and rubble. The nave is aisled with N and S porches, all of the 15thc and 16thc. and built of roughly coursed local stone with a granite plinth, and the chancel is of stone rubble, also with a granite plinth. The only Romanesque piece in the church is a Norman font of a type common in Cornwall, known as the Altarnun group. The church was restored by E. Sedding in 1867.

Altarnun does not appear in the Domesday Survey but is assumed by Gilbert, following Hals, to be taxed under Trewen, which would mean that it was held by Berner from the Count of Mortain in 1086. Alternatively it could have been included with Penpont, held by Osfrith from the Count. In either case, the rectory was given to the Cluniac monastery of Montacute, where it remained despite Mortain's heir William's forfeiture under Henry I, on account of his support for Henry's brother Robert for the crown. The Prior of Montacute transferred his rights in the church to the Bishop of Exeter in 1236.

The so-called Altarnun group of fonts includes Launceston, Jacobstow, Lawhitton, Laneast and Bratton Clovelly. The fonts at Callington, Warbstow and Ashwater (Devon) should also be included, although the motifs on the faces of these three differ from the standard 6-petalled rosette. It is also worth noting that the stone type varies between members of the group.

The Western Morning News contained an account of the reopening of the church after Sedding's restoration, which noted that the font was given a new granite step and a granite base, lined with lead and supplied with a flat oak lid.