The son of Denis and Adele Millett was educated at Harrow School, London and Trinity Hall, Cambridge, where he received a Master of Arts in classics and law in 1954. From 1955 to 1957, he served as Flying Officer in Royal Air Force.
Millett was called to the Bar at Middle Temple in 1955. In 1959, he came to Lincoln's Inn, where he was appointed a Bencher in 1980. From 1958 to 1986 he practised at the Chancery Bar and was examiner and lecturer in practical conveyancing at the Council of Legal Education from 1962 to 1976. Between 1967 and 1973, Millett was junior counsel at the Department of Trade and Industry in chancery matters, and between 1971 and 1975 member of the General Council of the Bar.
He was a member of the Law Commission working party on co-ownership of the matrimonial home in 1972 and 1973 and appointed a Queen's Counsel in the following year. From 1977 to 1982, Millett was member of the Department of Trade Insolvency Law Review Committee, and became a judge of the High Court of Justice in 1986, a post he held until 1994. He was a Lord Justice of Appeal between 1994 and 1998 and has been a non-permanent judge of the Court of Final Appeal, Hong Kong since 2000.
Millett was knighted in 1986 and became a member of the Privy Council in 1994. From 1991 to 1995, he was President of the West London Synagogue. On 1 October 1998, he was appointed as Lord of Appeal in Ordinary, receiving additionally a life peerage with the title Baron Millett, of St Marylebone in the City of Westminster. He retired as a Lord of Appeal in Ordinary in January 2004.
Lord Millett has been married to Ann Mireille Harris since 1959. They have two sons, Richard and Andrew, and five grandchildren, Edward, Alexia, Eliza, James and Isabella