The origins of Christianity in this part of Wales can be traced to around 530A.D. when St. Teilo laid the foundations of what is now Llandaff Cathedral and a few years later in 537A.D. one of his pupil monks, Isan travelled eastwards.   He established a religious settlement or 'Llan' at 'The Oval', north of Roath Park's wild gardens. 

St Isan's church dates back to the middle of the twelfth Century when after the Norman Conquest the Houses of Augustinian canons established The Abbey of Keynsham who awarded land in Llanishen as part of its Cardiff estate.  Robert Fitz Hamon in the 1080s allowed Tewkesbury Abbey to establish St Mary's Church and Priory in Cardiff, this resulted in Llanishen church being built as a chapel of ease to serve the tiny hamlet of Llanishen.

Following the Dissolution of the Monasteries in 1538 St. Isan's became the parish church of Llanishen.  It was at this time that the Lewis family of Van near Caerphilly, one of the local gentry was to become a main benefactor of St Isan's Church.  The Lewis family home was near the church at 'Llanishen House', they later moved to a mansion built in the 18th century on Thornhill known as 'New House'.

The Church building of St Isan Llanishen is of the English style of architecture originally with white-washed interior, formed from the original Norman Church with its 15th Century Tower.  The South Aisle was the former Nave, and the Lady Chapel the Sanctuary. It remained as such through the centuries, until minor alterations in 1854 and 1872, followed by a major building work in 1908, which formed and created the present Church building.

On the construction of the railway in 1871 an increase in the population of Llanishen was expected, and the Vicar at the time, Rev. Thomas Rees, increased the accommodation to 182 by building an aisle on the north side of slightly greater width than the old Church, which, however, made the building one-sided, and destroyed its proportion.