Llywelyn was born in 1173, the grandson of Owain Gwynedd. His father having been disposed of by Owain's other sons after Owain's death, Llywelyn was left to fend for himself until, as a youth, his natural superiority made itself felt and he defeated his uncles to take Gwynedd for himself. He consolidated this conquest in 1205 by marrying Joan, illegitimate daughter of King John of England. Despite occasional disputes with John and his successor, Henry III of England, Llywelyn succeeded in maintaining Welsh independence. After a long struggle, he gained the ascendancy over his main rival, Gwenwynwyn of Powys so as to unite the country.
His marriage to Joan has an unusual history. Following the birth of a legitimate heir, Dafydd ap Llywelyn, and a daughter, Elen (who was married off to the Norman Earl of Chester), Joan committed adultery with William de Braose or Breos, a Norman noble of south Wales who had allied himself with Llywelyn by the marriage of his daughter, Isabella, to Llywelyn's son, Dafydd. On learning of the affair in 1230, Llywelyn executed de Braose and Joan was imprisoned. Some time later, she was forgiven and restored to her position as princess, dying in 1237.
Llywelyn died in 1240 and a power struggle arose between his legitimate son, Dafydd, and his older, illegitimate son, Gruffydd, who according to Welsh law had equal rights of inheritance. Llywelyn had departed from tradition by naming Dafydd as heir, because he recognised the inherent flaws of the Welsh system. Gruffydd was killed attempting to escape from the Tower of London in 1244, leaving the field clear for Dafydd, but Dafydd himself died without heirs in 1246, and was eventually succeeded by his nephew, Llywelyn the Last.