The first reference to St Mawgan, the patron saint of this church, occurs in the 10th century. Thought to be one of the Welsh missionaries who brought Christianity to Cornwall, dedications to this saint are found at St Mawgan in Pydar and in Brittany. He may be depicted in the keystone of the tower window. The rounded shape of the original churchyard also indicates an early and important site.
Most Cornish churches started out as a chancel and nave to which transepts were added in the 13th or 14th centuries to make a cruciform church. Transepts were intended for burials and these could have included the two life-size effigies of a crusader, Sir Roger Carminow, who died in 1308, and his wife Joanna (though some say these came from Carminow chapel near Gunwalloe). The north transept was long used by the Vyvyan family of Trelowarren, but is now the vestry. The nave and transepts have fine wagon roofs, and some 15th or early 16th century work remains.
The tower is one of the finest in this part of the country with ashlar granite facing stone, though unbutressed; the belfry contains six bells. The west door is very fine with foliage carving and rather regal male and female heads at its base like Constantine. Coats of arms on the tower include the Reskymer family of Merthen in Constantine and also the Ferrers family of Bere Ferrers in Devon. Carminows and Ferrers intermarried and the latter church contains jewel-like 14th century glass commemorating this alliance. On the keystone of the tower arch, the master mason appears to have carved his tools. The squint, or hagioscope, in the north-east corner of the south transept opens up the view to the high altar from the congregation. This and the pillar in the middle of the transept opening could suggest that a south aisle was being planned but was aborted because of the Reformation which changed religion from Catholic to Protestant. Additional altars and the internal processional routes to them (aisles) now became a thing of the past. The south transept is used as a chapel for weekday services.
At the east end of the north aisle are the tombstones of the two first Vyvyan baronets, Sir Richard and Sir Vyell. Sir Richard was a very active Royalist in the Civil War, and built and garrisoned a fort on Dennis Head at his own expense. He also ran the Royalist mint in Truro producing silver coins for the king. The helmet and part of the sword that he used hang on a pillar nearby. .
Viewed from outside the east window of the north aisle has some particularly fine carving and is probably of 1520s or even later date. The carved granite jambs have been acclaimed as being ‘without parallel in the diocese’. The south doorway is probably of similar date with foliage carving and a niche (now empty) for the statue of a saint. Also a dog-flap, whose reason remains a mystery!