She was the daughter of Donald MacGylocher, the sixth Earl of Mar, and Helen of Wales. An educated woman, Isabella spoke Gaelic and High English. Her lands lay mostly to the northeast of Inverness.
Isabella was a wealthy young woman at the time of her arranged marriage to the Earl of Carrick, Robert Bruce. Her father was one of the seven guardians of Scotland who believed Robert Bruce to be the rightful King of Scotland. Despite the risks, he could see the advantage of the two families joining in marriage and bearing an heir to the throne. Mar was the first to sign over the estates of his family to the Bruce.
Isabella was married to Robert at the age of 18 and legend has it that they were much in love. Shortly after their marraige Isabella became pregant. She had a healthy pregnancy but she died soon after giving birth to a daughter, Majory in 1296.
Robert married his second wife, Elizabeth de Burgh, 6 years later.
Princess Marjory (d. 1316) carried on the legacy of the MacGylocher's through the female lines. She married Walter, High Steward of Scotland, and their son became Robert II of Scotland. From him descend the later later royal families of Scotland and England.