Joanna went to King's Landing in 259 AC, for the coronation of King Jaehaerys II Targaryen, and would remain as a lady-in-waiting for the future queen, Princess Rhaella Targaryen. There are rumours that Joanna had given her maidenhead to Prince Aerys the night of Jaehaerys' coronation, and briefly became Aerys' paramour after he ascended the throne himself. However, Grand Maester Pycelle insists these tales are baseless, as Tywin would not have married Joanna if they were true, "for he was ever a proud man and not one accustomed to feasting on another man's leavings."
Married life
Joanna married her first cousin, Tywin, in 263 AC in a lavish ceremony in the Great Sept of Baelor. On their wedding day, Tywin smiled, a very rare occurrence. However at the wedding feast, Aerys drunkenly japed about how it was a pity the first night tradition was banned, and he took certain "unwonted liberties" with Joanna in the bedding ritual when the men at the feast had to disrobe the bride.
Not long thereafter, Joanna was dismissed by Queen Rhaella from her service in King's Landing. While Queen Rhaella turned a blind eye towards most of her husband's infidelities, she did not approve when it concerned one of her own "ladies." (Joanna was one of many ladies to be sent away from court.) She departed for Casterly Rock at once, and seldom visited King's Landing afterwards.
Joanna's marriage to Tywin was reportedly a happy one. Joanna became Tywin's trusted counselor and companion. Her influence on her husband was such that people would say that Tywin ruled the Seven Kingdoms as Hand of the King, "but was ruled at home by his lady wife."
In 266 AC, Joanna gave birth to Cersei and Jaime, with Tywin present. King Aerys ordered Tywin to bring Joanna and the children to King's Landing when the children were old enough to travel, but Lord Tytos Lannister died before this could occur. Instead, Tywin, Aerys, Prince Rhaegar, and half the court went to the Westerlands in 267 AC, where they would remain for most of next year.
Joanna did attend the anniversary tourney in King's Landing in 272 AC, held to celebrate King Aerys' tenth year on the throne. There, King Aerys insulted Joanna, by asking her if nursing her children had ruined her breasts. Tywin, who was also present, was angered, and tried to resign as Hand of the King the next day, but King Aerys refused.
At some point within the next year, Joanna's servant caught Jaime and Cersei together, engaged in some kind of sexual activity. Joanna sent the maid away, moved Jaime's chamber to the other side of the castle, and put a guard outside of Cersei's chamber. She then warned her twins that they must never do such a thing again, or else she would be forced to tell their father. But it was not long thereafter, in 273 AC, that Joanna died giving birth to Tyrion.