The county is bounded to the north by the Irish Sea, to the east by Denbighshire, to the south by Cardigan Bay and Merionethshire, and to the west by Caernarfon Bay and the Menai Strait, which separates it from Anglesey. It has an area of 1,462 km². The surface is largely mountainous. A large part of the Snowdonia National Park lies in the county including Snowdon, the highest mountain in Wales at 1,085 m. The Ll%u0177n peninsula is less mountainous and contains many bays and sandy beaches. Bardsey Island is a major site for nesting seabirds. The River Conwy runs north along the eastern boundary, with Llandudno and Creuddyn to the north-east across the Conwy estuary being included in the county for historical reasons.

Principal towns are Bangor, Betws-y-Coed, Caernarfon, Conwy, Llandudno, Porthmadog and Pwllheli. Tourism, particularly on the coast, light industry, education and farming are the main industries, though the latter now only employs a small percentage of the workforce.

The Local Government Act 1888 created an elected Carnarvonshire county council in 1889, taking over functions from Caernarvonshire's Quarter Sessions. The administrative county covered by the county council had identical borders to the geographic county. The administrative county was formally renamed Caernarvonshire on July 1, 1926.

Under the Local Government Act 1972 the administrative county of Caernarfonshire was abolished on April 1, 1974. Caernarfonshire was largely split between the three districts of Aberconwy, Arfon and Dwyfor, both in the new county of Gwynedd along with Merionethshire and Anglesey. Since the Local Government (Wales) Act 1994 came into force on April 1, 1996 the county has been divided between the unitary authorities of Gwynedd to the west and Conwy to the east.