Lord Thomas was born and brought up in Cwmtawe. He read law at Trinity Hall, Cambridge (1966-69) and then as a Commonwealth Fellow at the University of Chicago Law School (1969-1970).. He was called to the Bar (Gray's Inn), began practising commercial law in 1972 and became Queen's Counsel in 1984 .In 1992, he was appointed to conduct the Department of Trade inquiry into the affairs of Mirror Group Newspapers when owned by Robert Maxwel.
He was appointed a High Court Judge in 1996 and was successively, a Presiding Judge in Wales (1998-2002), Judge in Charge of the Commercial Court in London (2002-03), a Lord Justice of Appeal (2003-2011), Senior Presiding Judge of England and Wales (2003-2006), Vice-President of the Queen's Bench Division and Deputy Head of Criminal Justice (2009-2011), President of the Queen’s Bench Division 2011-2013 and Lord Chief Justice of England and Wales 2013-2017. He was created a life peer in 2013, as Lord Thomas of Cwmgiedd.
Lord Thomas was a founder of the European Network of Councils for the Judiciary in 2003 (President, (2008-10) and of the European Law Institute in 2011 (serving on its governance until 2023 and First Vice President 2019-2023). He was Chairman of the Commission on Justice in Wales 2017-2019 reporting in 2019 “Justice in Wales for the People of Wales”.
He is President of the Qatar International Court; Chairman of the London Financial Markets Law Committee; Chair of the Steering Committee of the Standing International Forum of Commercial Courts (of which he was a founder); President of ARIAS (UK); Past President of the British Insurance Law Association, President of the London Shipping Law Centre and President of the Welsh Legal History Society (of which he was a founder in 1999). He practices as an international arbitrator and is an active member of the House of Lords.
Lord Thomas is Chancellor of Aberystwyth University, an Honorary Fellow of Trinity Hall, Cambridge, a Fellow of the Universities of Swansa, Bangor and Cardiff and the holder of Honorary Doctorates from other universities. He is a Fellow of the Learned Society of Wales. He has lectured on commercial, constitutional and other subjects around the world. He gave the 75th Series of Hamlyn Lectures in 2023 “Laws for a Nation and Laws for Transnational Commerce”.