Prior to the Roman invasion of 43AD, Devon was at the centre of one of Britain's most significant Celtic Kingdoms, that of the Dumnonia. This large, well founded kingdom incorporated all of Cornwall (Cornubia), Devon (Dyfneint), and much of West Somerset and West Dorset, with its original capital at Isca (Exeter). Modern Archaeology has indicated that this site was abandoned during the 5th Century, and, given the advances of the West Saxons, the capital would almost certainly have been continually relocated to the west. The main portion of Dumnonia in modern Devon (the Defnas, or "deep valley dweller" Britons) fell to Wessex around 650-670, while the Cornish remnant was still fully independent until 825-829. Although under Roman control until the late 4th century, the Celtic tribe of the Dumnonii probably retained a certain amount of self-government in their tribal lands (and may have been self-governed during Roman rule). They most probably re-established their kingdom as a power in its own right by the time of Magnus Maximus, as the latter prepared Britain's defences prior to establishing his own claim for control of the Roman Empire in AD 383, and Dumnonia was fully independent by AD410.

Devon takes its name from the Celtic word Dyfneint (meaning 'deep valley dwellers'), and even before the Romans came to Britain, the indigenous Celtic people had formed themselves into a number of independent nations, with the Dumnonii occupying Devon, Cornwall, and the western parts of Somerset and Dorset, whilst to the east lay the Durotiges and beyond the Belgae peoples. The Dumnonii capital is believed to have been located at Keresk ('Caer Uisc'), which the Romans were to call Isca Dumnoniorum, and which we now know as Exeter.

The Dumnonii were Iron Age Celts, but could certainly not be considered as savages. They had strong tribal traditions stretching back into the Bronze Age with no evidence of intermarriage with other tribes, and had reached a degree of civilization through their contacts with visitors seeking tin. By AD43, they were successfully mining tin and other minerals from Dartmoor, the Tamar Valley, and Cornwall, and were trading tin with early Mediterranean civilisations, including probably the Phoenicians, using the ancient port of Ictis (St. Michael's Mount, Cornwall). Whilst being noted as friendly towards strangers, they had at the same time established a reputation for being fiercely combative when threatened. Before the Roman invasion, conflicts between the independent tribal nations of pre-Roman Britain had led to the establishment of numerous hill forts along the tribal boundaries, and, with the Somerset levels then being largely tidally effected marshland, Dumnonia's only boundary to the east was relatively narrow and easy to defend.

After the Roman occupation, daily life seems little changed with the Dumnonian continuing to govern and to build new settlements in much the same manner as they had in the Iron Age. Neither did their style of building appear to have been influenced by the Romans, remained as it did wholly native in form. It would appear from this that the Dumnonii were set in their ways, and were not about to accept Roman influence readily. In fact there is some evidence that the Dumnonii accepted the Roman conquest without serious resistance once defeat was inevitable, and that as a result few garrison forts were placed in their territory. Certainly here is little evidence of Roman occupation west of the Exe, suggesting that the Romans had entered into a convenient alliance with Celtic Dumnonia. The Romans granted them civitas status (an administrative unit within a Roman province equivalent to a modern county) and the town of Exeter (Isca Dumnoniorum) was their administrative centre.

With the departure of the Romans in AD411 native rulers took over again, and in their final phase the Dumnonii held undisputed sway over the whole of the south-western peninsula until the Saxon kings of Wessex and Sussex ousted them from Devon early in the 8th century.