 He was the chief counsellor of Alexander III, King of Scots for the entire period of the king's majority and, as Scotland's leading magnate, played a key role in safeguarding the independence of the Scottish monarchy.During his long career, Alexander Comyn was Justiciar of Scotia (1258–89), Constable of Scotland (1275–89), Sheriff of Wigtown (1263–66), Sheriff of Dingwall (1264–66), Ballie of Inverie (in Knoydart) and finally, Guardian of Scotland (1286–89) during the first interregnum following the death of Alexander III.In 1284 he joined with other Scottish noblemen who acknowledged Margaret of Norway as the heiress to King Alexander.