Since 1974, it has been a civil parish governed by town council. The parish of Barnstaple has a population of 23,710 but the population of the urban area of the town taking in Sticklepath, Roundswell and Bickington is 30,916. Near-contiguous areas such as Bishop's Tawton, Fremington and Landkey and other satellite settlements make up a hinterland known as the 'Barnstaple Town Area' which has a population of 53,514.
The earliest settlement in the area was probably at Pilton on the bank of the River Yeo, now a northern suburb of the present town. Pilton is recorded in the Burghal Hidage (c. 917) as a burh founded by Alfred the Great, and it may have been the site of a Viking attack in 893, but by the later 10th century Barnstaple had taken over its role of local defence. Barnstaple had its own mint before the Norman Conquest.
The large feudal barony of Barnstaple had its caput at Barnstaple Castle. It was granted by William the Conqueror to Geoffrey de Montbray, who is recorded as its holder in Domesday Book. The barony escheated to the crown in 1095 after Montbray had rebelled against King William II. William re-granted the barony to Juhel de Totnes, formerly feudal baron of Totnes. In about 1107, Juhel, who had already founded Totnes Priory, founded Barnstaple Priory, of the Cluniac order, dedicated to St Mary Magdalene. After Juhel's son died without children, the barony was split into two, passing through the de Braose and Tracy families, before being reunited under Henry de Tracy. It then passed through several other families, before ending up in the ownership of Margaret Beaufort (died 1509), mother of king Henry VII. See Feudal barony of Barnstaple for full details.
In the 1340s the merchants of the town claimed that the rights of a free borough had been granted to them by King Athelstan in a lost charter. Although this was challenged from time to time by subsequent lords of the manor, it still allowed the merchants an unusual degree of self-government. The town's wealth in the Middle Ages was founded on its being a staple port licensed to export wool. It had an early merchant guild, known as the Guild of St. Nicholas. In the early 14th century it was the third richest town in Devon, behind Exeter and Plymouth, and it was the largest textile centre outside Exeter until about 1600. Its wool trade was further aided by the town's port, from which in 1588 five ships were contributed to the force sent to fight the Spanish Armada. Barnstaple was one of the "privileged ports" of the Spanish Company, (established 1577) whose armorials are visible on two of the mural monuments to 17th century merchants in St Peter's Church, and on the decorated plaster ceiling of the former "Golden Lion Inn", 62 Boutport Street (now a restaurant next to the Royal and Fortescue Hotel). The developing trade with America in the 16th and 17th centuries greatly benefited the town. The wealthy merchants that this trade created built impressive town houses, some of which survive behind more recent frontages—they include No. 62 Boutport Street, said to have one of the best plaster ceilings in Devon. The merchants also built several almshouses, and they ensured they would be remembered by installing elaborate monuments to their families in the church.
By the 18th century, Barnstaple had ceased to be a woollen manufacturing town, but this business was replaced by the import of Irish wool and yarn, for which it was the main landing place; the raw materials were carried by land to the new clothmaking towns in mid- and east Devon, such as Tiverton and Honiton. However, the harbour was gradually silting up—as early as c. 1630 Tristram Risdon reported that "it hardly beareth small vessels"—and Bideford, which is lower down the estuary and benefits from the scouring action of the fast flowing River Torridge, gradually took over the foreign trade.
Although for a time between 1680 and 1730, Barnstaple's trade was surpassed by Bideford's, it retained its economic importance until the early 20th century, when it was manufacturing lace, gloves, sail-cloth and fishing-nets, it had extensive potteries, tanneries, sawmills and foundries, and shipbuilding was also carried on.
Barnstaple was one of the boroughs reformed by the Municipal Reform Act 1835. Between the 1930s and the 1950s the town swallowed the villages of Pilton, Newport, and Roundswell through ribbon development.