The name "Macnab" (however spelt) is from the Gaelic "Mhic an Aba" and means "sons (or children) of the Abbot." Originally there were lay abbots, and according to tradition the Macnab chiefs were descended from the younger son of Kenneth McAlpine, King of the Scots, Abbot of Glendochart and Strathearn, who united the Scots and the Picts. Macnabs are members of a larger clan grouping, Siol-an-Alpine Clan Alpine, with the MacGregors, MacKinnons, Grants, Macquarries, and MacAulays.

The early history of Clan Macnab is bound up with Saint Fillan, the later of two so named. He was a Scot, the son of Feradach or Feriach, who succeeded St. Mundus as Abbot of Kilmun, then moved to Glendochart. The ruins of his chapel are at Kirkton in Strathfillan; his "pool" and "stone bed," supposed to cure the insane, are still there. Other relics of St. Fillan, important to the Clan, still exist. His pastoral staff, or crozier, (the Quigrich), which was carried before the Clan in battle, and his bell are in the National Museum in Edinburgh. His "healing stones" are at the Tweed Mill, Dochart Bridge, Killin. He died on 9 January, 703 A.D.

Macnab country stretched from Tyndrum west into Argyll, and east down Glendochart to Killin, where the seat of the Clan was Macnab Castle on Eilan Ran, an island on the north bank of the River Lochay. This was at the western end of Loch Tay, a point of great importance when there were no roads and water was the quickest means of transport.

The surname "Macnab" was first found in a document dated 1124 AD in the reign of David I. Angus Macnab, incensed by the murder of his brother-in-law, The Red Comyn, by Robert the Bruce, joined the Red Comyn's son-in-law, MacDougall of Lorn, and defeated the Bruce at the Battle of Dalrigh in Strathfillan. The Bruce then defeated MacDougall and Macnab at the Pass of Brander, 1308, and Bannockburn, 1314. The Macnab lands were forfeited, but in 1336, Gilbert of Bovain received a charter from King David II, and is regarded by the Lord Lyon as the first chief.

The Macnabs moved slowly down the River Dochart. Before Bannockburn, the Chief probably lived at Innishewan; Gilbert, the first Chief, lived at Bovain. About 1400, Macnab lands included Ardchyle, Invermonichele, Bovain, and Downich. The eighth Chief, Finlay, who died in 1525 at Eilan Ran and is buried at Killin, granted lands of Ewer and Leiragan to his wife, Mariat Campbell, for her lifetime. His eldest son was probably killed at Flodden in 1513.