Of ancient origin, the MacLachlan clan is descended from Niall of the Nine Hostages, High King in Ireland in 400 A.D.

Lochlainn was the name of the senior branch of the descendants of Niall, and by the thirteenth century the name appears in Scotland when Lachlan Mor of this family lived on the shores of Loch Fyne. Lachlan gave his name not only to the clan, but also to Lachlan Water, Lachlan Bay, the village of Strathlachlan and Castle Lachlan.

Aodh Anrothan (Hugh the Solitary), the son of Aodh Athlone O'Neill, traveled to Scotland from Ireland in the early eleventh century. He married the heiress of Cowal and Knapdale, a princess from the Royal House of Argyll. It is from this union that the MacLachlans descend.

1132. Gillecrist mac Finguni and Gillecrist mac Cormoic witnessed several Grants made in behalf of the Abbeys of Paisley and Lennox.

1164. According to tradition, Clan MacLachlan supported Somerled in his stand against the Scottish Crown. Somerled, the most powerful of the Warlords in the mid-twelfth century, led a revolt of the Argyll clans against the Crown. The clans considered themselves independent rulers of their region without obligation to the Crown.

Late 1100s. Suibhne Ruadh (the Red-haired Sween) is often mentioned in various pedigrees as being Toisech of Knapdale. He is credited with being the builder of Castle Sween, whose ruins still stand in grandeur on the shores of Loch Sween.