He educated at Fielding Agricultural High School and Victoria University of Wellington graduating with a BA and LLB in 1956. He was admitted to the bar as a barrister and solicitor of the Supreme Court (later the High Court) of New Zealand.
Following many years as a partner at New Zealand Law firm Russell McVeagh, he became a barrister sole and was appointed a QC in 1981.
In 1989-1990 he was President of the New Zealand Bar Association, and in 1990 he was appointed to the bench of the High Court of New Zealand. In 1995 he was elevated to the Court of Appeal. He retired from the appellate bench in 2001. As a judge some saw him as a proponent of judicial activism. He was also noted for his frequent dissenting judgements, particularly after 1996.
In 2002 Thomas was Visiting Fellow at the Research School of Social Sciences, The Australian National University, Canberra. From August 2004 he has been serving a two year appointment as Distinguished Visiting Fellow, The University of Auckland.
He was appointed a director of the Reserve Bank of New Zealand in 2003, for a five year term.[1]
Justice Thomas was brought out of retirement to become an acting Judge of the Supreme Court of New Zealand from 2005.
He was a noted author, his works including The Judicial Process: Realism, Pragmatism, Practical Reasoning and Principles (Cambridge University Press, Cambridge, 2005).
Thomas was made a member of the Privy Council in 1996. In 2002 he was appointed a KNZM.