The hill above Calstock commands a long bend of the River Tamar and a wide view of the valley. The Romans recognized its value first and built a fort there, the existence of which only became known to archaeologists in the early 21st century when University of Exeter excavations identified its outline. By that time the site had been a Christian parish church for more than seven hundred years. Tradition holds that the original building was consecrated about 1290, though nothing obvious of that 13th-century work survives. The pillars and arches on the north side of the centre aisle in the present church date from the early 14th century. Around 1420 the south aisle was added and the entire structure re-roofed. Most of the larger granite blocks in the tower and porch belong to that 15th-century campaign. The tower itself - three buttressed and battlemented stages, with battered pinnacles that were knocked down in a 1790 storm reported in the Exeter Mercury - is the most visible landmark for miles.

Attached to the east end of the north aisle is a small extension built in 1558 from a different stone than the rest of the church, slightly paler, slightly more crisply cut. This is the Edgcumb Chapel, family chapel of Richard Edgcumbe of Cotehele - the initials R.E. appear on the hood mould of the door. Inside, two late-17th-century monuments mark the chapel's role as a family burial place: one to Piers Edgcumb, who died in 1666, and one to Jemima, Countess of Sandwich, who died in 1674. The Edgcumbe family had been the dominant landowners on the Cornish side of the Tamar since the late medieval period. Their main house, Cotehele, sits a mile downriver in a fold of the wooded valley. The chapel is a small architectural document of that long territorial presence.

The south porch of the church preserves three unusual features that any visitor with five minutes to spare can examine. In the northeast corner are the worn remains of a holy water stoup, where parishioners once dipped their fingers before entering. In the southwest corner is something rarer: a fireplace, with granite lintel and jambs, set into the porch wall. A handful of comparable porch fireplaces survive in other Devon churches, but they are unusual enough that no one is quite sure what they were for. The Easter fire may have been kindled there. More plausibly, given the building dates from a period when fevers were thought to be airborne contagions, it was kept burning to ward off disease. The stone threshold contains the embedded remains of brass nails, suggesting the slab once held a brass effigy and was perhaps repurposed from an altar tomb. In the second stage of the tower, a large west-facing stained glass window is surmounted by four heraldic crests; the first is the arms of the Scorrer family of nearby Harewood.

By 1861 an architectural survey of the Diocese of Exeter found the building in need of major work. The thorough restoration that followed took place in 1867 at a cost of £600 and was directed by James Piers St Aubyn, an architect with offices in London and Devonport and a long list of West Country churches to his name. He was related to the St Aubyn family of St Michael's Mount. At Calstock he altered the floor levels, laid the existing tiles, gave the chancel its present roof, and installed plain pitched-pine pews - a heavy hand by modern conservation standards but, by Victorian church-restoration standards, relatively respectful. The older features survived. The 15th-century rood staircase turret on the north wall still climbs up through the masonry. The earlier granite quoins that marked the original end of the north aisle still register the building's growth.

The hilltop is now set in quiet rural countryside, but in the 19th century the valleys around it were dense with mines, quarries, brickworks, lime kilns, and shipyards. The parishioners of Calstock did most of those jobs, and many of them died at them. The gravestones in the churchyard record industrial accidents alongside the usual run of births and deaths. The adit of an unsuccessful copper mine called Wheal Trelawny still runs underneath part of the churchyard - a hollow under the dead. At the eastern end stands a granite cross marking the grave of Sir William Lewis Salusbury-Trelawny, 8th Baronet of nearby Harewood, who was Lord Lieutenant of Cornwall and M.P. for East Cornwall and died at Harewood in 1856 aged 75. Folklore says he refused to let his body leave his estate and was carried in through the back gate to lie as close to his own land as possible. Inside the church, two plaques north of the pulpit name the children of Sir William and his wife. The eldest of the first four lived to 28. A fifth child reached middle age. And outside again, Harry and Shadrach Gale are remembered on a stone, their names cut into Cornish granite, their bodies somewhere in the cold deep water between here and the Colorado they never returned to.