The fortress of Dumbarton was the stronghold of the kingdom of Alclud until the Vikings destroyed the fortress after a four-month siege in 870. This led to the emergence of the new kingdom of Strathclyde. The title "king of the Britons of Srath Clúade" was first used in 872. Dumbarton was later the county town of the county of Dumbartonshire. The name comes from the Scottish Gaelic "Dùn Breatainn" meaning "fort of the Brythons (Britons)". Alexander II granted the status of royal burgh in 1222.

Dumbarton Castle has the longest recorded history of any stronghold in Scotland. It sits on a volcanic plug of basalt known as Dumbarton Rock.

Mary, Queen of Scots stayed at Dumbarton Castle in July 1563. After the defeat at the Battle of Langside in 1568 she tried to reach the Castle, but went instead to England. John Fleming, 5th Lord Fleming, keeper of the Castle went with her into England and was allowed to return. When William Kirkcaldy of Grange governor of Edinburgh Castle changed sides to support Mary, this became a problem for Regent Moray. The subsequent conflict is known as the Marian Civil War. Fleming's defence of Dumbarton for Mary was staunch, assisted by the timely arrival of supply ships from France. The castle was captured by the forces of Regent Lennox in 1571, who used ladders to scale the rock and surprise the garrison.

Today not much survives from the medieval castle: the 14th-century Portcullis Arch, the foundations of two towers. There is a 16th-century guard house. Most of the existing structures were built in the 18th century, including the Governor's House.