[5] Williams' family moved to Sydney when he was a young child, and he attended St Ives High School.

[7] Williams then worked for a year as a solicitor at the law firm Blake Dawson Waldron (now Ashurst Australia), in Sydney.

[7] His first book was Australian Constitutional Law and Theory: Commentary and Materials, co-authored in 1996 with Tony Blackshield and Brian Fitzgerald, now in its seventh edition.

[15] News magazine Crikey reported that Williams had published a favourable opinion piece commending then Attorney-General Nicola Roxon,[15][16] who would later that year need to recommend appointment of a new High Court judge.

[22] In 2009, while on sabbatical from UNSW, Williams unsuccessfully sought pre-selection as the Labor candidate for the Canberra electorate of Fraser.

[23] Williams has been a vocal critic of aspects of anti-terrorism legislation passed following the September 11 attacks, such as control order regimes.

[26] Williams is married to Emma, a UNSW academic working in the field of corporate law, with whom he has two children: Edward and Ellie.