It was a maritime county, that was bounded to the north by the Irish Sea, to the east by Flintshire, Cheshire and Shropshire, to the south by Montgomeryshire and Merionethshire, and to the west by Caernarfonshire.

It was formed from Cantrefi taken as follows; From the Lordship of Denbigh: From Powys Fadog: In the south and west of what was Denbighshire, the mountains of the Clwydian Range rose from 1000 to 2,500 ft (760 m) high.

Two civil parishes: Llaneilian yn Rhos and Llansanffraid Glan Conway were administered as part of Conwy Rural District in the neighbouring county of Caernarfonshire.

In 1935 the rural districts were reorganised by a County Review Order, and reduced to five in number: Aled, Ceiriog, Hiraethog, Ruthin and Wrexham.

The administrative county was abolished in 1974, with most of its territory becoming part of the new districts of Colwyn, Wrexham Maelor and Glyndŵr in Clwyd.