When Robb Stark calls the Stark bannermen in response to his father Eddard Stark being imprisoned and the Lannister invasion of the Riverlands, the Greatjon is among the vassals who come to Winterfell. Lord Umber questions why he should support Robb, at which Robb threatens to hang him as a traitor if he doesn't. When Jon pulls out a greatsword, Robb's direwolf Grey Wind attacks him, biting off two fingers. The Greatjon laughs this off, Robb having won his support, and becomes Robb's champion.
When Robb splits his army the Greatjon goes with him to attack the forces of Jaime Lannister, "the Kingslayer", in the Battle of the Whispering Wood and the Battle of the Camps, the Greatjon burning Jaime's siege towers. His son Smalljon Umber is one of the young nobles who acts as Robb's personal guard. Robb frees Riverrun of the Lannisters and captures the Kingslayer. At Riverrun the nobles debate what to do after hearing of Eddard's execution by Joffrey Baratheon and Renly Baratheon's attempt to usurp the Iron Throne. The Greatjon proclaims Robb King in the North.
At the Red Wedding the Freys try to get Jon Umber too drunk to fight. However the Greatjon drinks Merrett Frey and Petyr Frey unconscious. When the massacre begins he snatches a sword from the first man to assault him, breaking their arm. It takes eight men to put him in chains, and he kills one, wounds two, and bites the ear off Ser Leslyn Haigh. His heir tries to protect Robb, but is beheaded by the forces of Robb's treacherous bannerman Roose Bolton, who personally murders his king.
The Greatjon and other nobles are held captive at the Twins by the Freys. His uncles Mors Umber and Hother Umber have been left as co-castellans of Last Hearth. Half of the remaining Umber forces, led by Mors, join Stannis Baratheon when he invades the North to defeat the Boltons. The others under Hother reluctantly join the Boltons due to his nephew being held captive, though Roose is uncertain of their loyalty.