The clan MacEwen trace their descent from the Kings of Ireland and claim a common ancestor with the Lamonts, MacLachlans and the MacNeils--Anradan. Together, these clans possessed most of the district of Cowal. The MacEwens were established on a strip of land along Loch Fyne in Argyll.

The earliest chief of the clan on record was Eoghain na' h' Oitrich (Ewen the Otter) who lived at the beginning of the 13th century. He was then succeeded by Severan II of Otter. Gillespie V of Otter assumed the chiefship about 1315 and was followed by Ewen, John and Walter. In 1432, the barony of Otter was confirmed by a charter of James I and was given to Swene MacEwen with a destination to the heirs of Duncan Campbell of Loch Awe. Swene MacEwen is the last chief on record.

After Swene's death in 1513, King James V confirmed the barony on Colin, Earl of Argyll. After that the lands of Loch Fyne remained in the possession of the Campbells. In 1598 many MacEwens were described as broken Highland men who were heavily armed and living by robbery. In 1602 the MacEwans were made subjects of the Earl of Argyll by an Act of Parliment.

The ruins of MacEwen's castle can be seen on the coast of Loch Fyne.