He succeeded his father in 1191 as Prince of Northern Powys. The Chronicle of the Princes of Wales records that "Madog son of Gruffudd Maelor died and was…buried in the monastery of Llanegwestl which he had previously founded" in 1236. 1
With his brother Owen, he succeeded Gruffydd in 1191 and, on Owen 's death in 1197, became sole ruler of Powys north of the Rhaeadr and the Tanat. Under his son, Gruffydd Maelor II, this area, comprising Welsh and English Maelor, Iâl, Cynllaith, Nanheudwy, and part of Mochnant, became known as Powys Fadog, in contrast with Powys Wenwynwyn.
At first friendly with Llywelyn ap Iorwerth, Madog deserted his cousin when the latter's fortunes were at a low ebb in 1211. He continued to hold aloof after Llywelyn, in 1212, had re-formed the Welsh confederacy, being treated as an official ally in king John 's pay. By 1215 he had adhered finally to Llywelyn's cause, remaining faithful to the end.
After his death in 1236 the unity of Powys Fadog was disrupted by partible succession among his five sons, Gruffydd Maelor II, Gruffydd Iâl, Maredudd, Hywel, and Madog Fychan. He was buried at his own foundation of Valle Crucis, the last Cistercian monastery to be founded in Wales. 2