As a youth he was slow and listless but grew into a courageous and ambitious leader who won Gwynedd and Powyss by battle in 1039 and defeated the Mercians. He contested with Gruffydd, son of Rhydderch ab Iestyn, over Deheubarth; but by 1055 he was the king of all Wales. About 1056 he married Ealdgyth, daughter of Aelfgar, Earl of Mercia, and they became the parents of three children.
There was a lengthy struggle with Harold Godwinson and, when Harold attacked his court at Rhuddlan through the treachery of his own men, Gruffydd ap Llywelyn was killed in 1063. His widow then married Harold Godwinson who, as King Harold II of England, in turn was killed in 1066 in the Battle of Hastings.