The ward of Llanishen and Thornhill is located in the north of Cardiff and is made up of suburbs of Llanishen in the south and Thornhill in the North. The Caerphilly Road forms the western boundary and the Llanishen Resevoir is on the eastern boundary.

The tallest building in Cardiff North and one of the city's major employers, HMRC Tax Office is located in Llanishen, as is the Welsh language broadcaster, S4C.

Llanishen and Thornhill is represented on Cardiff City Council by four Conservative Councillors