St Marcellus? Church is situated within a suboval churchyard, which appears to have formerly been larger and more curvilinear, encompassing an area to the north of the current road. The church is noted to be located some 300m south-east of two former round barrows. The church was garnted to Slebech Commandery in 1291. The `Merthyr? place-name element is thought to denote early medieval origins. The curvilinear churchyard may have originally been larger and it may lie within two concentric double-ditched outer enclosures. The church was a parish church during the post-Conquest period, belonging to the Deanery of Pembroke. It was granted to the Knights Hospitaller of Slebech during the later 12th century and is first mentioned in 1231, in a confirmation of the grant by John, son of Raymond by Bishop Anselm. The first recorded Vicar of Martletwy was Peter Filiol in 1317.

The church is a Grade 2 listed building, constructed of limestone rubble. It consists of 3-bayed nave, 1-bayed chancel, 3-bayed north aisle and south porch.
The church is medieval although heavily restored, but also retains an interesting early font and a fine early monumental effigy. During the 1840s it appears the roof was renewed. In the 1850s interior repairs were carried out and the seating improved. Work on the chancel was carried out in 1879, during which the tomb effigy of Sir Philip, an early fifteenth century priest, was discovered.The main restoration of the church, was completed in 1894 in the time of the Rev. F. O. Thomas. The work was to the designs of the architect Ernest V Collier of Carmarthen, at a cost of £805 toward which the ICBS granted £25. The windows were entirely restored and the roofs, floor and interior surfaces renewed to an extent which has left almost no historic detail intact.

The walls are not battered except for a small part at the NW corner of the nave. The roof is slate with red ridge tiles, coped gables to all parts except the porch, and carved crosses at all apexes.The doors and most of the windows are the work of the late nineteenth century restoration. Both the porch inner door and the door to the vestry have pointed arches. The windows of the nineteenth century restoration generally are pairs of lights with a top quatrefoil, in plate tracery. The restored bellcote for a single bell at the West end of the aisle is in similar masonry, but the arch over the bell is in brickwork.

Sources include:
Cadw Listed Buildings database
Cambria Archaeology, 2000, Pembrokeshire Churches, gazetteer, 48
Cambria Archaeology, 2003, Early Medieval Ecclesiastical Sites Project, Pembrokeshire gazetteer


NJR 10/01/2011; N Vousden 10 October 2018