Madog consolidated the possessions of his father,Gruffud Maelor, and the territory he ruled became known as Powys Fadog in his honour. (Fadog is a gender mutation of his name, Madog). Under his son, later, Gruffydd II ap Madog, this area comprising Welsh and English Maelor, Ial, Cynllaith, Nanheudwy and part of Mochnant formed Powys Fadog, as opposed to Powys Wenwynwyn and was still referred to as Powys Fadog although it was divided up between his five sons.

Madog was close to his cousin,Llywelyn ap lorwerth, initially, but gradually distanced himself and also kept aloof from 1212 when his cousin had managed to reform the Welsh Confederacy and looked instead to King John of England, in whose pay he was, as an official ally of the English King.

By 1215 he decided to ally with his cousin and remained so.

He had married Esyllt (Isota). He had issue:
    Gruffudd Maelor II, who succeeded his father,
    Gruffudd Iâl, died 1238.
    Maredudd, died 1256.
    Hywel, died 1268.
    Madog Fychan, died 1269.
    Angharad.

Madog ap Gruffydd Maelor died in 1236 and is buried at Valle Crucis Abbey, his own foundation, and the last Cistercian monastery to be founded in Wales.

In 1956 his heraldic slab was excavated at the Abbey.

He is either a great grandfather or great-great-great grandfather of Owain Glydŵr

Owain Glyndŵr, or Owain Glyn Dŵr, anglicised by William Shakespear as Owen Glendower and also sometimes styled Owain IV of Wales (c. 1354 or 1359 – c. 1416) by modern historians, was a Welsh ruler and the last native Welsh person to hold the titlePrince of Wales. He instigated an ultimately unsuccessful but long-running revolt againstEnglish rule of Wales.

Glyndŵr was a descendant of the Princes of Powys from his father Gruffydd Fychan II, hereditary Tywsog of Powys Fadog and Lord of Glyndyfrdwy, and of those of Deheubarth through his mother Elen ferch Thamas ap Llywelyn. On 16 September 1400, Glyndŵr instigated the Welsh Revolt against the rule of Henry IV of England. Although initially successful, the uprising was eventually put down — Glyndŵr was last seen in 1412 and was never captured nor tempted by royal pardons and never betrayed. His final years are a mystery.

Glyndŵr has remained a notable figure in the popular culture of both Wales and England, portrayed in Shakespeare's play Henry IV, Part 1 (as Owen Glendower) as a wild and exotic man ruled by magic and emotion ("at my nativity, The front of heaven was full of fiery shapes, Of burning cressets, and at my birth The frame and huge foundation of the earth Shaked like a coward." — Henry IV, Part 1, Act 3, scene 1). In the late 19th century the "Young Wales" movement recreated him as the father of Welsh nationalism, revising the historical image of him and joining him in popular memory as a national hero on par with King Arthur.