Originally wooded farm land, in A.D. 535 two monks came eastwards from the small religious settlement of
Llandaff, aiming to establish new settlements, or "llans", in the land below
Caerphilly Mountain. With fresh water from the Nant Fawr stream, one of the monks, Isan, founded his llan on the site of the modern day Oval Park. In 1089 at the
Battle of the Heath, the
Normans fought the Welsh Celts north of the settlement. The victorious Normans expanded Llanishen, starting work on a church to the north which was completed in the 12th century. Although
Oliver Cromwell had ties with Llanishen and the neighbouring village of
Lisvane, the village remained undisturbed until 1871, when the
Rhymney Railway was given permission to break the stranglehold of the
Taff Vale Railway into
Cardiff Docks. Building a line from
Caerphilly to Crockherbtown Junction just north of
Cardiff Queen Street, its tunnel to the north resulted in the accidental deaths of a number of people, many of them buried in St Isan's church. The railway and the development of
Llanishen railway station allowed wealthy Cardiff businesspeople to commute from the village to the city centre easily, resulting in the expansion of the village's population by 20,000 between 1851 and 1871. In 1887, the two new reservoirs of Llanishen were built to allow distribution of water collected in the
Brecon Beacons to the city. In 1922, after expansion north by the city and south by the village, Llanishen became a suburb of Cardiff.
Llanishen Golf Club was established in 1905. Development of the village since has been through redevelopment of former farming and military land into commercial usage and housing development.
Thornhill was part of the Llanishen civil parish until November 2016, when a new community of Thornhill was created north of the Linear Park. ==Commerce and industry==