Before the Romans it was the capital of a British (Celtic) kingdom., and endured as “British” (rather than of the Picts or Scots) through and after the Roman occupation of England. Tradition has the 4th/5th century St. Patrick being born there. During the dark ages it expanded its control of much of south-east Scotland and became capital of the kingdom of Strathclyde before being taken over by the Scottish king Malcolm II and absorbed into a unified kingdom by Duncan I in 1034.
Dumbarton Rock, which rises from the shore was the site of fortifications since the Iron Age. During the Middle Ages it became a royal castle. William Wallace was imprisoned there before he was taken to London to be executed.
The town gave its name to the mansion in Washington DC, “Dumbarton Oaks”, which hosted the international conference in 1944 that prepared for the creation of the United Nations. Earlier, a popular Dumbarton Oaks concerto for orchestra was commissioned from Igor Stravinsky as an anniversary present by the owner, Robert Woods Bliss.