Lord Wyman Manderly was Lord of White Harbor and one of principal participants in the restoration of the united North, following the banishment of Lord Roose Bolton and restoration of House Stark to the dominance over the region. Furthermore, he served for ten years as the Regent and Warden of the North for Lord Rickon Stark, before he reached adulthood.
He took a part in the War of the Five Kings, staunchly standing by King Robb I Stark during its course; however in the brutal event, which occurred after the wedding ceremony of Lord Edmure Tully and Roslin Frey and was known as the Red Wedding ever since, he lost a second son, Wendel, and with his firstborn son being held by the Lannisters, his position in the North had been substantially weakened and he had to submit to the rule of Roose Bolton, the newly appointed Warden of the North.
Slowly chewing on that sour bite, Lord Wyman enrolled the help of Lord Davos Seaworth in an effort to bring the youngest Stark boy, Rickon, back home from the islands of Skagos, and set out for the wedding of fake Arya Stark to Ramsay Bolton to Winterfell.
He participated in the Battle of Ice and helped the victory of King Stannis Baratheon, who, nonetheless, had him after numerous squabbles imprisoned for the murder of Lord Davos, which was in truth feigned. The northern lords, spurred by the deed, carried out a successful try on King Stannis's life weeks later. In eight moon of 300 AC, Lord Davos returned from Skagos along with Rickon Stark. Without further hesitations, Lord Wyman had him proclaimed the true Lord of the North, and appointed himself his Regent and Warden of the North.
Lord Wyman later led the attack on Dreadfort, which ended with Lord Roose Bolton's surrender and banishment to the Night's Watch. His newborn son and heir to Dreadfort is taken to Lord Wyman's wardship as the northern host marches back to Winterfell.
Meanwhile, King Aegon VI marries Lady Sansa Stark, Lord Rickon's older sister, and based on this alliance Lord Rickon along with Lord Wyman decide to swear fealty to the Iron Throne. This occurs during the eleventh moon of 300 AC.
Some times later, Lord Manderly is confronted by Lord Commander Jon Snow, who is urgently requesting help in the War for the Dawn. Lord Manderly as Warden of the North decides to march forth the northern host, including his personal levies.