The church is mostly of late medieval appearance, with two arcades, clerestory and high aisle walls. The chancel was rebuilt, apparently on old foundations, in 1859, the W tower also being of this date, formerly there only being a bellcote. A Romanesque tympanum is preserved on the inside of the modern chancel. There is also a broken tub font of indeterminate date.

Altham was a dependent chapelry in the vast parish of Whalley around the Ribble valley. It is possible it is one of the two churches recorded in Whalley's entry for the Domesday Book, as it is along with the mother church, the only one of the chapels to preserve Romanesque fabric.

Unlike many claimed "ancient fonts", the tub font does not appear to be a misidentified agricultural trough or similar: the lesenes appear to be a modest attempt at ornament and the shape is completely wrong for such a function. It should be noted that the church has a smaller late-medieval font at the other end of the aisle with a rope-moulding at the bottom of the stem (such a motif is highly unusual for Perpendicular style fonts, but as it is monolithic with the rest of the font, it cannot be reused Romanesque).