Lord John Thomas was a practising barrister in England and Wales until 1996, a Judge of the High Court and Court of Appeal of England and Wales (1996–2013) and then Lord Chief Justice of England and Wales (2013–2017). He was President of the European Network of Councils of the Judiciary (ENCJ) (2008–2010).

He is Chairman of the Welsh Government’s Commission on Justice in Wales, Chairman of the UK Financial Markets Law Committee, Chancellor of Aberystwyth University (from January 2018), an Honorary Fellow of Trinity Hall, Cambridge and a Fellow of the Universities of Bangor, Cardiff and Swansea. He is one of the ELI’s founding members and a member of the ELI Executive Committee.