During the Roman occupation of Britain, the area, that later became known as Devon, was the dominion of the Celtic tribe of the Dumnonii, the "Deep Valley Dwellers". It was a somewhat remote region of the province, less Romanized than others. An area of tin mining, unpopular with the rich Roman farmers who built elaborate villas in adjoining Somerset. The major Roman influence was from the army, whose base at Isca Dumnoniorum (Exeter) grew into a major Roman city with all the grand buildings and public amenities associated with such. It is possible that, towards the end of the period of Roman rule in Britain, a leading family from amongst the Dumnonii, under one Caradoc, was already taking a part in the administration of the local 'civitas'. His descendants certainly claimed as much and, when in AD 410 the Roman army deserted the province, this group gathered enough support to make themselves Kings of Dumnonia. Their kingdom - which at its greatest extent covered modern Devon, Somerset and Cornwall - flourished for nearly five hundred years. However, Anglo-Saxon invasions were a constant threat as these people took over the old Celtic regions further east. There were many battles with the West Saxons who, by AD 658, had taken most of Somerset. They appear to have gained a strong foothold in Devon around the early 680s, when the Kings of Dumnonia probably withdrew to safer strongholds in Cornwall. They, however, still attempted to influence the area for the next hundred years and it Devon was not formally annexed by Wessex until AD 805. The 'pagus' of Dumnonia was called Dyfneint by the Saxons, an early form of the name which survives as Devon today. The area is first mentioned in the Anglo-Saxon Chronicle in AD 823 and it was soon given an Ealdorman to act on behalf of the king in the region. His men are recorded as having beaten off Viking Invasions in AD 851 & 878, though Exeter was successfully sacked in 1003. Devon was originally part of the Wessex Sees of Winchester and then Sherborne but, in AD 905, it was given its own Bishop, based at Bishop's Tawton, though quickly transferred to Crediton. Bishop Leofric moved the diocenal centre to Exeter shortly before the Conquest.

After King William the Conqueror's successful invasion of Britain, in 1066, he quickly recognised the importance of securing the West Country. He besieged Exeter for eighteen days before honourable terms were agreed for its surrender. Like the rest of the country, the rich Devon farmland was divided up amongst William's Norman Barons. Chief amongst these 'honours' were Plympton, Okehampton, Barnstaple, Harberton and Totnes. The descendants of these men became famous Devon families. Plympton was bestowed on the Redvers in the 12th century along with the Earldom of Devon. Thse later passed to the Courtenay family, who also possessed Okehampton. The Dukedom of Exeter was given to the Hollands in the 14th century, but they became extinct in the male line in the reign of Edward IV. The ancestors of Sir Walter Raleigh, who was born and grew up at East Budleigh, also for long held considerable estates in the county.

Devon had an independent sheriff, originally hereditary, but later appointed for one year at a time. In 1320, however, it was complained that the local hundreds were in the hands of great Lords who did not appoint enough bailiffs to keep the county in proper order. Devon returned two members of parliament in 1290, but this had increased to six (Barnstaple, Exeter, Plympton, Tavistock, Torrington & Totnes), only five years later. There were 26 members by 1832, when the Reform Act reduced them to 18. Subsequent bills have brought about further reductions. Devon miners always somewhat independent from the county administration and had their own 'Stannary Courts' for the regulation of mining affairs around the four stannary towns of Ashburton, Chagford, Plympton and Tavistock. The ancient parliament of the miners used to meet, in the open air, on Crocken Tor. The tin mines pre-date the Romans but, by the 14th century, there is additional mention of copper, lead, gold and silver. At the time of the Domesday Survey (1086), the salt industry was important in the county, which also had at least 99 mills and 13 fisheries. Cloth has always been the chief manufacturing industry and a statute of Edwrad IV's reign allows for the production of distinctive types in particular areas of Devon. Cider making is mentioned by the 16th century and Lace making (now famous in Honiton) was undertaken at Colyton and Ottery St. Mary from 1680.

Many armed conflicts have impacted on Devon over the centuries. In 1140, during the Civil War of King Stephen's reign, Baldwin de Redvers held out against the King at his castles in Exeter and Plympton. Coastal attacks by the French were a frequent occurrence in the 14th and 15th centuries and there were often local skirmishes in the local area during the Wars of the Roses: the Earl of Devon being a Lancastrian and Lord Bonville, a Yorkist. After the Battle of Lose-Field in 1470, King Edward IV pursued his cousin, Warwick 'the Kingmaker' and his brother, the Duke of Clarence, as far as Exeter; where they met up with the latter's wife and fled to the Continent via Dartmouth. Richard III personally visited Devon to counteract rebellious rumblings and declared the Bishop and Dean of Exeter to be outlaws. The Royal pretender, Perkin Warbeck, besieged Exeter in 1497 and Henry VII later sat in judgement on the rebels there. More famous in the West, however, is the Prayer Book Rebellion of 1549. Following the Reformation of the Church of England by Henry VIII - unpopular in Devon - there were serious disturbances in the county upon the introduction, by his son, Edward VI, of the Protestant Prayer Book. A priest at Sampford Courtenay was persuaded to read the old Mass the very next day and the insubordination quickly spread. Protest turned to open revolt, joined by also the men of Cornwall. Exeter suffered an appalling siege but was eventually relieved by Lord Russell.

Devon mostly favoured Parliament at the outbreak of the English Civil War but a prevailing desire for peace brought about a treaty for the cessation of hostilities in both Devon 7 Cornwall in 1643. Skirmish did continue for a while, until the final capture of Dartmouth and Exeter in 1646. After the Monmouth Rebellion, Judge Jefferies held a 'Bloody Assize' at Exeter and, in 1688, the prince of Orange (later William III) began the Glrious Revolution by landing at Torbay and moving towards London through Forde and Exeter.