Lord of the Eyrie and Defender of the Vale, Jon Arryn was Warden of the East and Hand of the King during Robert Baratheon's reign. In his youth, Jon was the Keeper of the Gates of the Moon until his father, Lord Jasper, died. Jon was married to Jeyne Royce until her death without producing an heir. The same fate befell his second wife and cousin Rowena Arryn.

Jon Arryn was one of the principal leaders of Robert's Rebellion, raising his banners to protect his squires Robert and Eddard. To secure the support of Lord Hoster Tully in the rebellion, Jon married a third time to Lysa Tully. After Robert took the Iron Throne, he named Jon the Hand of the King. As Hand, Jon brokered peace with Prince Doran Martell to stop Dorne from revolting in the name of Prince Viserys Targaryen. Over Robert's fifteen year reign, Jon was left with much of the responsibility of running the Seven Kingdoms. He was a close friend and father figure to both King Robert and Lord Eddard through most of their lives.

Before his death, Jon worked with Stannis Baratheon to discover that Queen Cersei's children were fathered not by Robert but by the Queen's brother Jaime. Before he could publicly reveal this information, he was poisoned by his wife Lysa on the instigation of Petyr Baelish.