Dumbarton was formerly the county town, and the county was originally also spelled Dumbartonshire. The County Council, set up under the Local Government (Scotland) Act 1889 adopted the spelling 'Dunbarton' at the start of the 20th century. Dumbarton is thought to derive from the Gaelic Dùn Breatainn (Fort of the Britains), but the town stuck with the name Dumbarton, and some people continue to refer to the county as Dumbartonshire.

The county retained a large exclave despite the boundary changes in the 1890s elsewhere in Scotland, containing Kirkintilloch and Cumbernauld, between Stirlingshire and Lanarkshire : this area had originally been part of Stirlingshire, but had been annexed to Dunbarton in the reign of David II at the request of Malcolm Fleming, Earl of Wigtown, the owner of the land, who was also Sheriff of Dumbarton.

Dunbartonshire (Siorrachd Dhùn Bhreatainn in Gaelic) or the County of Dumbarton, is a lieutenancy area and a registration county of Scotland. Between 1890 and 1975 it was a county. The area had been previously been part of the historic district of Lennox, which was a duchy in the Peerage of Scotland.

The county was abolished in 1975 by the Local Government (Scotland) Act 1973, when it became part of the large Strathclyde Region. Strathclyde was divided into nineteen districts, with the area of the former county being divided between Dumbarton, Bearsden and Milngavie, Clydebank, Cumbernauld and Kilsyth and Strathkelvin Districts, the latter also containing a small part of the former Lanarkshire.

The regional identity was retained for some major functions such as fire service and police at the next reorganisation of local government in 1996, but for most purposes the former county then found itself served by three new unitary councils: Argyll and Bute Council (which took over the Helensburgh and Lomond part of Dumbarton District), West Dunbartonshire Council and East Dunbartonshire Council.