Lord Thomas attended Rugby School, then studied law at Trinity Hall, Cambridge, and at the University of Chicago. He was called to the Bar by Gray's Inn in 1969, and became a member of 4 Essex Court (now Essex Court Chambers), where he practiced commercial law. He was appointed Queen's Counsel in 1984, and was a Recorder from 1986 to 1996, when he was made a Judge of the Queen's Bench Division. He was nominated to sit in the Commercial Court, and was was Judge In Charge from 2002 to 2003. He was a Presiding Judge on the Wales & Chester Circuit from 1998 to 2001. He was appointed Lord Justice of Appeal in 2003, President of the Queen's Bench Division in 2011, and Lord Chief Justice of England & Wales in 2013. After retiring from the Bench in 2017, he returned to practice as a commercial arbitrator and became President of the Qatar International Court.

Lord Thomas is a keen gardener, walker, and traveller.