In 537 A.D. a pupil monk of St. Teilo named Isan established a religious settlement or “Llan” at “The Oval” Llandennis Road, which is north of the Wild Gardens at Roath Park. Later in 1080 Tewkesbury Abbey established St. Mary’s Church and Priory in Cardiff and Llanishen church was built as one of the chapels of ease to service the tiny hamlet of Llanishen. Today’s church has been built on previous church foundations going back to the 12th century.
St. Isan’s Church originally consisted of a Nave, which is now the South Aisle, and a Sanctuary, which is now the Lady Chapel. It then acquired a 15th century tower and then remained as such until minor alterations were undertaken in 1854. Then, with the arrival of the railway in 1871, alterations were undertaken in 1872 to increase the seating capacity to 182. This was accomplished by building another aisle on the north side of the building which is now the Nave. Major building works then took place in 1908 to form the church as it stands today. In 2008 to celebrate the 100th anniversary of the last extension a total refurbishment was undertaken, new lighting was installed, the pews were stripped of their dark stain and the interior repainted.