Clan Maclachlan claims descent from Lachlan Mor, who lived on the shores of Loch Fyne in the 13th century. Lachlan belonged to the family who originally emigrated from Ireland to Scotland in the 11th century.

The progenitor of this family, Anrothan, son of Aodh O'Neil, king of the north of Ireland, is said to have married the heiress of the King of Scots and gained lands campaigning there. Moncreiffe wrote that it was more likely Anrothan married a local king of Argyll or a sub-king of Cowal. Through this marriage, Anrothan's descendants gained control of the lands of Knapdale and Cowal, and several Scottish clans claim a descent from him including Clan Lamont, Clan MacEwen of Otter, Clan MacNeil of Barra, and the MacSweens who became the Irish Sweeney Clan who left Scotland and returned to Ireland in the 14th century as leaders of Gallowglass.