Angus Og was the second successor to his father Angus Mor MacDonald. His brother Alexander first inherited and on Alexander's death, then Angus Og succeeded them in 1308. During his brothers life, they had both been in favor of the Isles being held under the English crown. Archibald MacDonald in his book, The clan Donald, states that there is a letter written by Angus that states that he and Hugh Bisset had been with the English fleet at the island of Bute and that he and Bisset were awaiting the King of England's commands as to whether they should attack the Lord of Lorn and asks that the king form his own opinion as to whether the Lord of Lorn was loyal to England. He appeared to be loyal to the King of England until the time when Robert the Bruce was forced to take refuge in his lands. After this time he was a supporter of Robert I the Bruce. He seems that he must have truly befriended the Bruce, because at the time he provided aid and shelter to him, Bruce was hiding from the English and there was no benefit for Angus Og to help him.
When Alexander III died, his heir was Margaret, The Maid of Norway. She was Alexander's granddaughter by his daughter Margaret. Margaret had married Eric II of Norway and most likely died giving birth to Margaret, the Maid. When she died on the voyage from Norway to Scotland in 1290.
Previous to this time there existed a dispute over the succession to the throne, between Robert Bruce and the Balliols for the throne. Alexander II had forced them to sign an agreement that his granddaughter would be the heir to the throne.
During the struggle for Scotland's independence, was when Robert the Bruce was forced to seek refuge with Angus Og, who as previously stated willingly gave it to him. Angus hid him in his castle of Saudell in Cantirey, and later moved him to his fortress of Dunvertie on the island of Mull and then when this became unsafe moved him again to the island of Rathlin. At the battle of Bannockburn, Angus Og provided ten thousand men, which he brought to the field with him. Angus and his Islemen were placed on the right flank of Bruce as a permanent mark of distinction for their gallantry in battle. During this time of war in Scotland the MacDoughall's relatives of the MacDonald's and lords of Lorn, were supporters of the Comyns and Balliols, being connected to these families by marriage. This opposition of Bruce caused them to forfeit their lands. Bruce gave the isles of Mull, Jura, Coll and Tiree to Anges Og. These lands had previously been disputed by Angus Og and the MacDoughalls. Angus was also given half of the lordship of Lochaber, which had been forfeited by the Comyn family.
Angus Og married a woman from the family of O'Cahan in Ulster, variously referred to as O'Cathan, or O'Kane. Her first name was probably Agnes, because it was recorded on a Chancery Roll, A.D. 1338, where Agnes, the mother of John of the Isles is granted a safe-conduct protecting her when crossing the Channel fro Ireland as often as she wished to do so. She apparently returned to Ireland to live after her husband's death. Archibald MacDonald says her father was Guy or Conbuidh O'Cathan, one of the great barons of Ulster, Lord of Limvady, and master of the whole County of Derry.
Angus received as part of her dowry, seven score of men, from all of the different surnames under O'Cathan and he settled them on his newly acquired lands.
Angus Og was described as a "personable, modest man,affable,and not disaffected to king or state. His son by O'Cathan's daughter, was named John and he succeeded him to the throne.
Angus Og died at Isla at his palace of Finlagan in the year 1325 and was interred in Icolumbkill inside the chapel of St. Oran in Iona. And his King Robert Bruce died shortly before.