Clan Donnachaidh, which means the children of Duncan, emerged from the union of the Scottish and Pictish kingdoms and counts among its forebears the kindred of St. Columba.

Abbot Crinan of Dunkeld, descended from the kindred of St. Columba, was father of Duncan, King of Scots. He was killed by MacBeth, but his descendants held the throne for two and a half centuries. The king had a younger son, Maelmare, who became Earl of Atholl and was the ancestor of the chiefs of Clan Donnachaidh.

The chiefs are numbered from Duncan the Stout (stout in battle rather than in belly), who lived in the 1300s. He held lands in Rannoch and around Glen Errochty and took his followers to fight at Bannockburn in 1314 in support of his friend King Robert Bruce. His son Robert (perhaps called after Bruce) inherited land from his own mother and his estate ran from the edge of the Grampians to the gates of Perth.

The main clan name and that of the chiefs is Robertson, but many other names taken by those descending from the chiefs or belonging to families who lived on clan estates and sought the chief's protection can be considered septs of the clan.