The MacEwens, like the MacNeills, are descended from Aodh Anrothan (Hugh the Solitary), who relocated from Ireland to Scotland in the early eleventh century, and married an heiress of Cowal and Knapdale. Four generations later, one of his descendants, Suibhne (Sween) held Castle Sween, the "Key to Knapdale."

What with one thing and another, in the next century, the lands of Knapdale went to the Earl of Monteith; the MacSweens backed King Edward of England in the Scottish War of Independence, and therefore were dispossessed by Robert the Bruce. In the 1300s, another descendent, now called "Ewen" had established a base on Loch Fyne, in land between that of the Lamonts and the MacLachlans. At this point, the head of the clan was known as "MacEwen of Ottir."

In 1432, Swene MacEwen, 9th chief of Clan MacEwen, granted his lands to Duncan Campbell of LochAwe, and resigned his Barony of Otter to the King of Scotland. The displaced MacEwens became a protectorate of the MacLachlans.

The crest, of the present McEwen baronet of Bardrochat in Ayrshire, is the stump of a cut down oak tree, sprouting forth young branches. The motto, REVIRESCO, means "I will grow again."