The islands may correspond to the Cassiterides (Tin Isles) visited by the Phoenicians and mentioned by the Greeks. However, the archipelago itself does not contain much tin.
It is likely that until relatively recent times the islands were much larger and perhaps joined together into one island named Ennor. Rising sea levels flooded the central plain around 400–500 AD, forming the current 55 islands and islets, if an island is defined as "land surrounded by water at high tide and supporting land vegetation". The word Ennor is a contraction of the Old Cornish En Noer (Moer, mutated to Noer), meaning 'the land' or the 'great island'.
Evidence for the older large island includes:
- A description written during Roman times designates Scilly "Scillonia insula" in the singular, indicating either a single island or an island much bigger than any of the others.
- Remains of a prehistoric farm have been found on Nornour, which is now a small rocky skerry far too small for farming. There once was an Iron Age Britain community here that extended into Roman times. This community was likely formed by immigrants from Brittany, probably the Veneti who were active in the tin trade that originated in mining activity in Cornwall and Devon.
- At certain low tides the sea becomes shallow enough for people to walk between some of the islands. This is possibly one of the sources for stories of drowned lands, e.g. Lyonesse.
- Ancient field walls are visible below the high tide line off some of the islands (e.g. Samson).
- Some of the Cornish language place names also appear to reflect past shorelines, and former land areas.
- The whole of southern England has been steadily sinking in opposition to post-glacial rebound in Scotland: this has caused the rias (drowned river valleys) on the southern Cornish coast, e.g. River Fal and the Tamar Estuary.
Offshore, midway between Land's End and the Isles of Scilly, is the supposed location of the mythical lost land of Lyonesse, referred to in Arthurian literature, of which Tristan is said to have been a prince. This may be a folk memory of inundated lands, but this legend is also common among the Brythonic peoples; the legend of Ys is a parallel and cognate legend in Brittany as is that of Cantre'r Gwaelod in Wales.
Scilly has been identified as the place of exile of two heretical 4th century bishops, Instantius and Tiberianus, who were followers of Priscillian.
Norse and Norman period
, who visited the islands in 986. It is said an encounter with a cleric there led him to Christianise Norway
In 995, Olaf Tryggvason became King Olaf I of Norway. Born c. 960, Olaf had raided various European cities and fought in several wars. In 986 he (supposedly) met a Christian seer on the Isles of Scilly. He was probably Priscillian and a part of the tiny Christian community that was exiled here from Spain by Emperor Maximus for Priscillianism and their part in the Priscilline heresy. In Snorri Sturluson's Royal Sagas of Norway, it is stated that this seer told him:
Thou wilt become a renowned king, and do celebrated deeds. Many men wilt thou bring to faith and baptism, and both to thy own and others' good; and that thou mayst have no doubt of the truth of this answer, listen to these tokens. When thou comest to thy ships many of thy people will conspire against thee, and then a battle will follow in which many of thy men will fall, and thou wilt be wounded almost to death, and carried upon a shield to thy ship; yet after seven days thou shalt be well of thy wounds, and immediately thou shalt let thyself be baptised.
The legend continues that, as the seer foretold, Olaf was attacked by a group of mutineers upon returning to his ships. As soon as he had recovered from his wounds, he let himself be baptised. He then stopped raiding Christian cities, and lived in England and Ireland. In 995, he used an opportunity to return to Norway. When he arrived, the Haakon Jarl was facing a revolt. Olaf Tryggvason persuaded the rebels to accept him as their king, and Jarl Haakon was murdered by his own slave, while he was hiding from the rebels in a pig sty.
With the Norman Conquest, the Isles of Scilly came more under centralised control. About 20 years later, the Domesday survey was conducted. The islands would have formed part of the "Exeter Domesday" circuit, which included Cornwall, Devon, Dorset, Somerset, and Wiltshire.
In the mid-12th century, there was reportedly a Viking attack on the Isles of Scilly, called Syllingar by the Norse, recorded in the Orkneyinga saga— Sweyn Asleifsson "went south, under Ireland, and seized a barge belonging to some monks in Syllingar and plundered it." (Chap LXXIII)
...the three chiefs—Swein, Þorbjörn and Eirik—went out on a plundering expedition. They went first to the Suðreyar [Hebrides], and all along the west to the Syllingar, where they gained a great victory in Maríuhöfn on Columba's-mass [9 June], and took much booty. Then they returned to the Orkneys.
"Maríuhöfn" literally means "Mary's Harbour/Haven". The name does not make it clear if it referred to a harbour on a larger island than today's St Mary's, or a whole island.
It is generally considered that Cornwall, and possibly the Isles of Scilly, came under the dominion of the English Crown late in the reign of Æthelstan (r. 924–939). In early times one group of islands was in the possession of a confederacy of hermits. King Henry I (r. 1100–35) gave it to the abbey of Tavistock who established a priory on Tresco, which was abolished at the Reformation.
Later Middle Ages and early modern period
Scilly was one of the Hundreds of Cornwall
(formerly known as Cornish Shires) in the early 19th century.
Scilly Isles: map by John Bartholomew (1874)
At the turn of the 14th century, the Abbot and convent of Tavistock Abbey petitioned the king,
stat[ing] that they hold certain isles in the sea between Cornwall and Ireland, of which the largest is called Scilly, to which ships come passing between France, Normandy, Spain, Bayonne, Gascony, Scotland, Ireland, Wales and Cornwall: and, because they feel that in the event of a war breaking out between the kings of England and France, or between any of the other places mentioned, they would not have enough power to do justice to these sailors, they ask that they might exchange these islands for lands in Devon, saving the churches on the islands appropriated to them.
William le Poer, coroner of Scilly, is recorded in 1305 as being worried about the extent of wrecking in the islands, and sending a petition to the King. The names provide a wide variety of origins, e.g. Robert and Henry Sage (English), Richard de Tregenestre (Cornish), Ace de Veldre (French), Davy Gogch (possibly Welsh, or Cornish), and Adam le Fuiz Yaldicz (Spanish?).
It is not known at what point the islanders stopped speaking the Cornish language, but the language seems to have gone into decline in Cornwall beginning in the Late Middle Ages; it was still dominant between the islands and Bodmin at the time of the Reformation, but it suffered an accelerated decline thereafter. The islands appear to have lost the old Celtic language before parts of Penwith on the mainland, in contrast to the history of Irish or Scottish Gaelic.
During the English Civil War, the Parliamentarians captured the isles, only to see their garrison mutiny and return the isles to the Royalists. By 1651 the Royalist governor, Sir John Grenville, was using the islands as a base for privateering raids on Commonwealth and Dutch shipping. The Dutch admiral Maarten Tromp sailed to the isles and on arriving on 30 May 1651 demanded compensation. In the absence of compensation or a satisfactory reply, he declared war on England in June. It was during this period that the Three Hundred and Thirty Five Years' War started between the isles and the Netherlands.
In June 1651, Admiral Robert Blake recaptured the isles for the Parliamentarians. Blake's initial attack on Old Grimsby failed, but the next attacks succeeded in taking Tresco and Bryher. Blake placed a battery on Tresco to fire on St Mary's, but one of the guns exploded, killing its crew and injuring Blake. A second battery proved more successful. Subsequently, Grenville and Blake negotiated terms that permitted the Royalists to surrender honourably. The Parliamentary forces then set to fortifying the islands. They built Cromwell's Castle—a gun platform on the west side of Tresco—using materials scavenged from an earlier gun platform further up the hill. Although this poorly sited earlier platform dated back to the 1550s, it is now referred to as King Charles's Castle.
During the night of 22 October 1707, the isles were the scene of one of the worst maritime disasters in British history, when out of a fleet of 21 Royal Navy ships headed from Gibraltar to Portsmouth, six were driven onto the cliffs. Four of the ships sank or capsized, with at least 1,450 dead, including the commanding admiral Sir Cloudesley Shovell.
There is evidence for inundation by the tsunami caused by the 1755 Lisbon earthquake.
The islands appear to have been raided frequently by Barbary pirates to enslave residents to support the Barbary slave trade.