He was appointed as a High Court judge on 3 October 1994, assigned to the Chancery Division, and received the customary knighthood.

On 20 December 2011, Carnwath was announced as an appointee to the Supreme Court of the United Kingdom.

[3] By Royal Warrant, all members of the Supreme Court, even if they do not hold a peerage, are entitled to the judicial courtesy title "Lord" for life.

Lord Carnwath is an honorary professor of law at University College London, an Honorary Fellow of Trinity College, Cambridge, and a visiting professor in practice at the Grantham Research Institute at the London School of Economics.

He is a former governor of the Royal Academy of Music (Hon FRAM) and former chairman of the Britten-Pears Foundation.