David Thomas qualified as a solicitor in 1969, later becoming: a Council member of The Law Society; a Committee member of the City of London Law Society; and President of Liverpool Law Society. In 1997 he was appointed Banking Ombudsman. On creation of the Financial Ombudsman Service in 2000 he became Principal Ombudsman. From 2009 to 2010 he was Chief Ombudsman.
He is now: a part-time consultant to the Financial Ombudsman Service; a non-executive director of the Legal Ombudsman; and service complaints reviewer for the Scottish Public Services Ombudsman. He has advised internationally on consumer protection in more than 20 countries, including for The World Bank, the European Commission and the United States Agency for International Development.