He attended the Cyfarthfa Grammar School and later read history at University College Wales, Aberystwyth.

Williams received his doctorate for a dissertation later published as Medieval London: from commune to capital.

He moved back to Wales in 1974, becoming Professor of History at University College Cardiff, where he stayed until his retirement in 1983.

After his retirement, he continued to write, but he focused more and more on television and film, presenting, with Wynford Vaughan-Thomas, a 13-part series in 1985 by HTV and Channel 4 on Welsh history entitled The Dragon Has Two Tongues.

[2][3] Williams was also a supporter of Republicanism; and later a member of Plaid Cymru, he praised the anti-monarchy book The Enchanted Glass by Tom Nairn.