Sir Glanmor was born in Dowlais, into a working-class family, and was educated at Cyfarthfa Grammar School in Merthyr Tydfil.
He studied at Aberystwyth alongside Alun Lewis and Emyr Humphreys, becoming a specialist in the early modern period of Welsh history.
In subsequent works, such as Owen Glendower (1966), Recovery, Reorientation and Reformation (1987), and Owain Glyndwr (1993) Williams documented how English subjugation of Wales was strengthened by the Tudor Acts of Union, and yet still offered scope for the growth and development of Welsh culture.
He wrote equally fluently in Welsh, with his best known works including: Dadeni, Diwygiad a Diwylliant Cymru (The Renaissance, the Reformation and the Culture of Wales, 1964), Grym Tafodau Tân (The Power of Fiery Tongues, 1984) and Cymru a'r Gorffennol: côr o leisiau (Wales and the Past: a choir of voices, 2000).
[3] Speaking after Williams' death in Swansea in 2005, the vice-chancellor, Richard B Davies, said: "his influence on the study of Wales is incalculable.