Castle Sween takes its name from Suibhne, who is thought to have built the castle. Suibhne is believed to have been a grandson of Hugh “the Splendid” O’Neill who died in 1047.
In the XIII century the Clan MacSween, or descendants of Suibhne, governed lands extending as far north as Loch Awe and as far south as Skipness Castle on Loch Fyne. In the later half of the XIII the MacSween lands of Knapdale passed into the hands of the Stewart Earls of Menteith.
By the time of the Wars of Scottish Independence the MacSweens took the wrong side, and when Robert the Bruce became King of Scots, he displaced the MacSweens from their lands. After Robert the Bruce had defeated MacDougall Lord of Lorne in 1308, he then laid siege to Alasdair Og MacDonald in Castle Sween. Alastair gave himself up and was disinherited by Robert Bruce who then granted Islay to Alasdair’s younger brother, Angus Og, the king’s loyal supporter, who also received the Castle Sween in Kintyre from the King.
In 1310, Edward II of England granted John MacSween and his brothers their family’s ancestral lands of Knapdale (though by then Castle Sween was held by Sir John Menteith). It is possible that this could be the “tryst of a fleet against Castle Sween,” recorded in the Book of the Dean of Lismore, which tells of the attack of John Mac Sween on Castle Sween.
In 1323, after the death of Sir John Menteith, the Lordship of Arran and Knapdale passed to his son and grandson. In 1376, half of Knapdale, which included Castle Sween, passed into possession of the MacDonald Lords of the Isles, by grant of Robert II of Scotland to his son-in-law John I, Lord of the Isles.
During the MacDonald’s century and a half of holding the castle, the castellans were first MacNeils and later MacMillans.
In 1490 Castle Sween was granted to Colin Campbell, 1st Earl of Argyll, by James IV of Scotland.
In 1647, during the Wars of the Three Kingdoms, Castle Sween was attacked and burnt by Alasdair MacColla and his Irish Confederate followers.