When Robb Stark called his banners in response to the arrest of his father, Lord Eddard, in King's Landing, Lord Karstark came with his forces and his three sons. They remained with Robb's army in the early stages of the War of the Five Kings, and Lord Karstark had to witness the death of two of his sons, Torrhen and Eddard, at the hand of Jaime Lannister during the Battle of the Whispering Wood. When news of Lord Eddard's execution reached the Northern camp and Greatjon Umber proclaimed Robb the King in the North, Karstark was among the first to swear loyalty to the new monarch.
When he learned that the king's mother, Lady Catelyn, had released Jaime Lannister from captivity in order to bargain for her daughters' lives, Lord Rickard was outraged. He and a few of his men murdered two young prisoners, Willem Lannister and Tion Frey, in their cells in Riverrun. In response to this transgression, King Robb declared him a traitor and sentenced him to death. Robb himself beheaded Lord Karstark, who cursed and denounced him with his last breath, calling him a kinslayer.