Eldest son of Lord Tymond Lannister and older twin to Ser Tyland, born at Casterly Rock late in the reign of King Jaehaerys I. With his twin he was a knight and a tilter of some renown, and in 112 AC the two brothers were sent to court Princess Rhaenyra Targaryen during her royal progress through the west, with neither one of them having any success. Jason succeeded his father as Lord of Casterly Rock under King Viserys I and wed Johanna of House Westerling, who bore him one son, Loreon, and three daughters, of whom Cerelle and Tyshara would later be put forth as possible brides for King Aegon III.
When the Dance of the Dragons broke out at King Viserys's death in 129 AC, Lord Jason declared his house for Queen Alicent's son Aegon II while his twin Tyland served as the king's master of coin in King's Landing. Jason led the host of the westerlands into the riverlands to strike at Rhaenyra's supporters from the west; at the Battle of the Red Fork in 130 AC he was struck down by the outlaw archer Pate of Longleaf, ever after called the Lionslayer for the deed. The remainder of his host was shattered shortly after at the Battle by the Lakeshore — the bloodiest battle of the civil war — and Casterly Rock thereafter passed to his four-year-old son Loreon under the regency of his widow Johanna.