The Gwentian Chronicle records that "his son…Grufydd" succeeded his father as prince of Gwynedd after "Llywelyn son of Seisyllt" was killed in 1021. His parentage is confirmed by the Chronicle of the Princes of Wales which records that "Bleddyn son of Cynvyn [and] Gruffudd son of Llywelyn…were brothers by the same mother…Angharad daughter of Meredudd king of the Britons". The Gwentian Chronicle records that "Grufudd" defeated and killed "Iago son of Idwal prince of Gwynedd" and "took the government of Gwynedd and so became king of Wales from the Irish Channel to the Severn sea", dated to  from the context.
He succeeded in 1039 as Prince of Gwynedd and Powys. The Chronicle of the Princes of Wales records that "Iago king of Gwynedd was slain" in 1037 and that "Gruffudd son of Llywelyn son of Seisyl governed in his stead", adding that the latter "from beginning to end pursued the Saxons and the other nations and killed and destroyed them…first…at Rhyd y Groes on the Severn". He accompanied Svein Godwinsson, son of Godwin Earl of Wessex, on an expedition into South Wales in 1046. He made himself ruler of all of Wales. Allied with Ælfgar ex-Earl of East Anglia, son of Leofric Earl of Mercia, he invaded England and sacked Hereford Oct 1055. Peace was negotiated by Leofric Earl of Mercia and Harold Earl of Wessex, under which Gruffydd swore allegiance to King Edward "the Confessor". Leofgar, recently appointed Bishop of Hereford, led an army against Gruffydd but was defeated and killed 16 Jun 1056. He started raiding England again after Ælfric Earl of Mercia died . England launched a full-scale campaign against him, led by Tostig Earl of Northumbria invading north Wales and Harold raiding the southern coast. The Gwentian Chronicle records that "Caradoc son of Rhydderch [error for son of Grufudd son of Rhydderch?] son of Iestin hired Harallt to come with an army to S. Wales" and together they defeated Prince Gruffydd in 1060.
The Chronicle of the Princes of Wales records that "Gruffudd son of Llywelyn…fell through the treachery of his own men" in 1061. Simeon of Durham records that "Griffin king of the Britons" was killed by his own men "Non Aug" in 1064. Gruffydd was not caught by this pincer movement but was killed by his own men. 1
Not much information about his youth is available but some traditions have been preserved in the tales of Walter Map. As a youth he is said to have been slow and spiritless, but ambition later turned him into a man of valour and boldness and developed in him imagination and steadfastness of purpose. After Iago ab Idwal had been slain by his own men in 1039 Gruffudd ap Llywelyn became king of Gwynedd and Powys. Immediately afterwards, he struck a blow against the Saxons of Mercia in the battle of Rhyd-y-groes on the Severn and drove them to flight. This victory made him a prominent figure; and thenceforth until his death he continued to be the shield of his country and the terror of its enemies. Having struck the men of Mercia and made the Marches safe he turned his attention to Deheubarth, where Hywel ab Edwin was king. There is not much information about the conflict between the two; but in 1040 Gruffudd again attacked Ceredigion and burned Llanbadarnfawr. In 1041 Gruffudd again defeated Hywel in the battle of Pencader, but he did not succeed in overpowering him completely for in 1042 Hywel defeated a host of ‘ Black Gentiles ’ at Pwlldyfach (today, Pwlldyfarch ), near Carmarthen. Two years later ( 1044 ), Hywel brought with him from Ireland a fleet of the ‘ Black Gentiles,’ but he was slain in a fierce encounter with Gruffudd in the estuary of the Towy. Even after this Gruffudd failed to gain possession of Deheubarth; Gruffydd ap Rhydderch ap Iestyn rose up against him. According to ‘ Brut y Tywysogion ’ ( Pen. MS. 20, 18a ), great deceit and treachery took place in 1045 between Gruffydd ap Rydderch and his brother Rhys and Gruffudd ap Llywelyn. The latter was forced to call in the help of Swegen son of Godwin, in his endeavour to uphold his authority in Deheubarth. In 1047 about 140 of Gruffudd's war-band were slain through the treachery of the men of Ystrad Tywi, and to avenge them Gruffudd plundered Dyfed and Ystrad Tywi; but that was all he succeeded in doing and for the next eight years Gruffydd ap Rhydderch was in sure possession of Deheubarth. Gruffudd ap Llywelyn directed his efforts in another direction; early in the summer of 1052 he invaded the land of Hereford and defeated a combined host of Saxons and Normans near Leominster. In 1055 he slew Gruffydd ap Rhydderch and thus at last gained possession of Deheubarth. Moreover, leagued with Aelfgar of Mercia he attacked the Saxons and Normans of Hereford under earl Ralph, drove them to flight and set fire to the town. Earl Harold was sent to avenge this attack but all he succeeded in doing was to repair Hereford and to come to terms with Aelfgar. In 1056 Leofgar, bishop of Hereford, led an army against Gruffudd, and on 16 June a battle took place between them in the valley of the Machawy. Once again Gruffudd was victorious. Then, through the efforts of earl Harold, earl Leofric of Mercia, and Ealdred of Worcester, an agreement was reached and Gruffudd swore fealty to king Edward. About this time too Gruffudd m. Ealdgyth, daughter of Aelfgar, and when Aelfgar was again exiled in 1058 Gruffudd, with the help of Magnus Haroldson, assisted him in regaining his lands. The close alliance between Gruffudd and Aelfgar ensured security for Wales, but towards the close of the year 1062, when Aelfgar had died, earl Harold without warning fell upon Gruffudd 's court at Rhuddlan, but Gruffudd made good his escape. In 1063 Gruffudd was slain ‘through the treachery of his own men,’ according to ‘ Brut y Tywysogion,’ after he had been ‘the head and shield and defender of the Britons.’ 2