William was born in 1933 and educated at Eton followed by National Service from 1951 to 1953. He graduated with a law degree from Trinity College Cambridge in 1956 and then won a Harkness Fellowship to study law at Harvard University. He has practised as a barrister since 1959 and was appointed Queen's Counsel in 1979.
In 1972 he joined the board of Justice, the British Arm of the International Committee of Jurists. During the referendum on Britain's membership of the EC he was an active member of "Lawyers for Europe" but remained uninspired by the existing Political Parties. When Roy Jenkins delivered the Dimbleby Lecture he joined the Council for Social Democracy and was a founder of the SDP. He was the drafter of its Constitution, a job he undertook again seven years later for the Liberal Democrats. He chaired the SDP conference committee from 1982-1988 and then the Liberal Democrat conference committee until 1991. He was a key supporter of the merger between the SDP and the Liberals. He served on the Federal Policy Committee 1988-1997 (latterly as vice-chair) and became an active member of policy working groups.
William was the parliamentary Candidate for Kensington 1983 and 1987 and for the byelection 1988 and also 1992 for Oxford West and Abingdon.
He was knighted in 1989 and then made a Life Peer in 1997. In the Lords he was the Party's shadow Lord Chancellor until December 2006, when he was appointed Chair of the Delegated Powers Committee. William also sat on the Committee on Standards in Public Life and two Select Committees.
William married Celia Herbert in 1966 and they have two daughters, one son and three grandchildren. His outside interests include, walking, skiing and foreign travel.