Justice Williams has an LLB from the Victoria University of Wellington and an LLM (Hons) from the University of British Colombia. He became a partner at Kensington Swan in 1992 and went on to co-found Walters Williams & Co in 1994.

In 1999 he became Chief Judge of the Māori Land Court and was appointed Deputy Chairperson of the Waitangi Tribunal shortly after in 2000.

In 2004, he was made Chairperson of the Waitangi Tribunal.

Justice Williams was appointed a judge of the High Court in September 2008.

A judge of the Court of appeal in February 2018.

In 2019 the first Māori judge of the Supreme Court.

In the 2020 New Year Honours, he was appointed a Knight Companion of the New Zealand Order of Merit, for services to the judiciary and brings a unique blend of legal intellectual rigour and tikanga Māori to his role.

Justice Williams iwi affiliations are Ngati Pūkenga, Waitaha and Tapuika.

Confessions of a native judge: Reflections on the role of transitional justice in the transformation of indigeneity (aiatsis.gov.au)