Arran has a particular concentration of early Neolithic Clyde Cairns, a form of Gallery grave. The typical style of these structures is that of a rectangular or trapezoidal stone and earth mound that encloses a chamber lined with larger stones labs. Pottery and bone fragments found inside the chambers suggest they were used for internment and some have forecourts, which may have been an area for public display or ritual. There are two good examples in Monamore Glen east of the village of Lamlash, and similar structures called the Giant's Graves above Whiting Bay. There are numerous standing stones dating from prehistoric times, including six stone circles on Machrie Moor (Gaelic: Am Machaire).

Several Bronze Age sites have been excavated, including "Ossian's Mound" near Clachaig and a cairn near Blackwaterfoot that produced a bronze dagger and a gold fillet. Torr a' Chaisteal Dun in the south west near Sliddery is the ruin of a Iron Age fortified structure dating from about AD 200. The original walls would have been 3 metres (9.8 ft) or more thick and enclosed a circular area about 14 metres (46 ft) in diameter.

Gaels, Vikings and the medieval era An ancient Irish poem called Agalllamh na Senorach, first recorded in the 13th century, describes the attractions of the island.

Arran of the many stags. The sea strikes against her shoulders, Companies of men can feed there. Blue spears are reddended among her boulders. Merry hinds are on her hills, Juicy berries are there for food, Refreshing water in her streams, Nuts in plenty in the wood.

The monastery of Aileach founded by St. Brendan in the 6th century may have been on Arran and St. Molaise was also active, with Holy Isle being a centre of his activities. The caves below Keil Point (Gaelic: Rubha na Cille) contain a slab which may have been an ancient altar. This stone has two petrosomatoglyphs on it, the prints of two right feet, said to be of Saint Columba.

In the 11th century Arran became part of the Sodor (Old Norse: 'Suğr-eyjar'), or South Isles of the Kingdom of Mann and the Isles, but on the death of Godred Crovan in 1095 all the isles came under the direct rule of Magnus III of Norway. Lagman (1103-1104) restored local rule. After the death of Somerled, Arran and Bute were ruled by his son Angus. In 1237, the Scottish isles broke away completely from the Isle of Man and became an independent kingdom, before being ceded to the Scottish crown in 1266 by the Treaty of Perth. A substantial Viking grave has been discovered near King's Cross south of Lamlash, containing whalebone, iron rivets and nails, fragments of bronze and a 9th century bronze coin, and another grave of similar date nearby yielded a sword and shield. Arran was also part of the medieval Bishopric of Sodor and Man.

On the opposite side of the island near Blackwaterfoot is the King's Cave (see above) where Robert the Bruce is said to have taken shelter in the 14th century. Bruce returned to the island in 1326, having earlier granted lands to Fergus MacLouis for assistance rendered during his time of concealment there. Brodick Castle played a prominent part in the island's medieval history. Probably dating from the 13th century, it was captured by English forces during the Wars of Independence before being taken back by Scottish troops in 1307. It was badly damaged by action from English ships in 1406 and sustained an attack by John of Islay, the Lord of the Isles in 1455. Originally a seat of the Clan Stewart of Menteith it passed to the Boyd family in the 15th century.