Presumed a younger son of Loren the Last and Lord of Casterly Rock through the long arc of the Faith Militant uprising against the kings of the dragons. Wed to Lady Jocasta of House Tarbeck, he resented the Lannisters' light footing under the new Targaryen regime and could do little of it openly. When the conflict broke out in 42 AC and Prince Aegon and his sister-wife Princess Rhaena were caught up in a royal progress through the westerlands, Lyman gave them refuge at Casterly Rock and refused to surrender them to King Maegor the Cruel, though he stopped short of supporting Aegon's claim by arms. He shrewdly played both sides: when his bastard son Ser Tyler Hill led five hundred men from Lannisport to join the Faith rebellion, Lyman plausibly denied having ordered it, while still letting his westermen ride against the king.
When Maegor died in 48 AC and Jaehaerys I took the throne, the Lannisters bent at once to the new king. Lyman led three hundred knights to the Golden Wedding the following year and hosted Rhaena and her husband Andrew Farman at Fair Isle for a time. King Jaehaerys, however, never wholly trusted the Lord of Casterly Rock, judging him an ambitious man who meant to use influence to edge into the inner circle of the Targaryens at the cost of the Velaryons and Baratheons. Lyman was twice passed over for the Mastership of Coin and once for the Handship, and he and many of his bannermen died from the Shivers in 59 AC.