Centuries before Tresillian church was contemplated , Merther parish had a church – now disused and falling into decay – some two miles away to the south of the Tresillian river dedicated to St Cohan; and centuries before this, St Cohan, a Briton of the 5th or 6th century lived and witnessed for God which is what the name Merther really means. His little well and consecrated chapel , now demolished, were close to that church site of Merther and near that site which was called “Scean field”, is a spring known as Holy Well.
The last relics were removed from there about the middle of the last centaury and most are now safely stored in Tresillian.
Until the 14th century Merther was a subordinate Chapelry to nearby Probus; some writers recorded it as such until the middle of the 19th century but such evidence as is available points to the probability that it was made parochial in the 15th century.
This present church of Tresillian was first erected in 1878 as a Chapel of Ease to Merther Church and was rebuilt and dedicated to Holy Trinity in 1904 when it became the parish Church of Merther and the neighbouring parish of Lamorran.
The three bells are all 18th century. Two came from Merther and the other from Lamorran.