Jeor Mormont was the Lord Commander of the Night’s Watch, before being murdered by several of the brothers of the Night’s Watch that had planned a mutiny. He was considered to be a great commander and had been seriously concerned about the depleting resources of the Night’s Watch. Before his death, he trained Jon Snow and offered his wisdom and experience.
Prior to joining the Night’s Watch, Jeor Mormont was lord of Bear Island, a remote island of the North. During Robert’s Rebellion, Jeor fought loyally alongside Ned Stark and Robert Baratheon and helped to win the Rebellion. Within a few years after, he abdicated his seat as Lord of Bear Island, so that his son, Jorah, could become Lord of Bear Island. Jeor, joined the Night’s Watch and within a few years, he rose to the position of Lord Commander.