[1] The son of Clement William and Marion Mustill, he was educated at Oundle School and St John's College, Cambridge, where he graduated with a Doctor of Laws in 1992.

He served in the Royal Artillery from 1949 to 1951, was called to the Bar, Gray's Inn in 1955, became a Queen's Counsel in 1968 and a Bencher in 1976.

He was made Chairman of the Civil Service Appeal Tribunal in 1971 and Recorder of the Crown Court in 1972, holding both posts until 1978, when he was knighted.

On 10 January 1992, he was appointed Lord of Appeal in Ordinary and in consequence created a life peer as Baron Mustill, of Pateley Bridge in the County of North Yorkshire.

Lord Mustill married twice, firstly in 1960 to Beryl Davies, they divorced in 1983; and secondly to Caroline Phillips in 1984, with whom he had two sons and one stepdaughter.