Comyn was from one of the most important noble families of Scotland. They were arch-rivals of Bruce for influence over the Scottish government. Comyn’s great-uncle was John Balliol, who had been chosen as king of Scotland back in 1292 and his father had been Guardian of Scotland jointly with Bruce from 1298-1300.
After his father was killed by Bruce in 1306, the young Comyn was taken in by Edward I. In 1314 he was eager to win back his family lands from Bruce and avenge his father’s death. Through his grandmother Eleanor Balliol, he also had a weak claim on the Scottish throne.
Comyn fought and was killed, probably fighting alongside his uncle the earl of Pembroke’s men as they escorted Edward II from the battlefield in defeat.