Tion was the second born son of Lord Gerold Lannister and Rohanne Webber. He was the younger twin of Tywald Lannister, the heir to Casterly Rock. He became the heir of his father when his twin died fighting in the Peake Uprising. Tion supposedly held his brother as he died. Tion also served as the squire of Prince Aegon Targaryen during the battle.
Tion had been betrothed to a son of Lord Rowan, but instead he asked his father whether he could marry Ellyn Reyne, who had been his brother's betrothed. Gerold opposed the match at first, but eventually he gave in and accepted the betrothal. Supposedly the cause was that Tion told Gerold that Tywald had pleaded for him to "take care of Lady Ellyn". Tion was married to Ellyn in the same ceremony that saw Tytos Lannister married to Jeyne Marbrand.
In 236 AC, Daemon III Blackfyre started the Fourth Blackfyre Rebellion. The war was shortlived, and culminated in the Battle of Wendwater Bridge. The royalists only lost 100 men during the battle, but Tion was among the deceased.
Tion's widow would try to seduce Tytos after her husband's death, but she was swiftly remarried to Lord Walderan Tarbeck. This brought the reign of the Reynes to an end.