John of Islay was the Lord of the Isles (1336–1386) and chief of Clan Donald. In 1336, he styled himself Dominus Insularum, although this was not the first ever recorded instance of the title in use. Some modern historians nevertheless count John as the first of the later medieval Lords of the Isles, although this rather broad Latin style corresponds roughly with the older Gaelic title Rí Innse Gall, in use since the Viking Age. For instance, the even more similar Latin title dominus de Inchegal, applied to Raghnall Mac Somhairle in the mid-12th century. In fact John is actually styled Rí Innsi Gall or King of the Isles shortly after his death in a contemporary entry in the Irish Annals of Ulster. Clan Donald considers the title "Lord of the Isles" to have been in use at least since Angus Mor Macdonald, who died in 1293, and the title "King of the Isles" in use since Somerled, the Norse-Gael who forged the Kingdom of the Isles in the 12th century.

Lord of the Isles or King of the Isles (Scottish Gaelic: Triath nan Eilean or Rìgh Innse Gall) is a title of Scottish nobility with historical roots that go back beyond the Kingdom of Scotland. It began with Somerled in the 12th century and thereafter the title was held by a series of his descendants, the Norse-Gaelic rulers of the Isle of Man and Argyll and the islands of Scotland in the Middle Ages. They wielded sea-power with fleets of galleys (birlinns). Although they were, at times, nominal vassals of the kings of Norway, Ireland, or Scotland, the island chiefs remained functionally independent for many centuries. Their territory included much of Argyll, the Isles of Arran, Bute, Islay, the Isle of Man, Hebrides, Knoydart, Ardnamurchan, and the Kintyre peninsula. At their height they were the greatest landowners and most powerful lords after the kings of England and Scotland.