Mang lived in the town ruled by Udilf. He was the father of Eeta.
Mang and Udilf had been business partners. Udilf had the ability to know what people needed, and Mang was a skilled craftsman. When the town needed a new leader, they chose Udilf. Mang disagreed with the decision, but treated Udilf with respect.
When a race of "creatures" began attacking nearby villages, Udilf prepared the town for battle. Mang was acting as gatekeeper when the Fifth Doctor and his companions arrived.
During dinner one evening, Mang entertained the townspeople with the story of how his father had fought the Romans. Tegan, knowing from her history that the Romans had been long gone at this point, argued with Mang about his facts, insulting him. When she went to apologise the next day, Mang had her clean the workshop.
Prior to the attack, Adric instructed Mang and the townspeople in the creation and use of longbows. When the "creatures" attacked the town, Udilf and Eeta were among the many who died. Mang took leadership of the survivors, and they headed for Wales, which Tegan had mentioned to Mang. (PROSE: The Immortals)
In an alternate timeline where Polly Wright was manipulated into interfering with temporal nexus points, Mang killed Adric by accident when he was distracted by Polly's sudden appearance. (PROSE: That Time I Nearly Destroyed the World Whilst Looking for a Dress)