As QC he appeared regularly for the Inland Revenue, and was leading counsel for the Revenue in the landmark cases of Ramsay v Inland Revenue Commissioners (1982) AC 300 and Furniss v Dawson  AC 474, in which he had the unique experience of citing American tax cases to the House of Lords. He also appeared in any leading cases in the House of Lords on contract law, notably Johnson v Agnew  AC 367 and Sudbrook Trading Estate v Eggleton  1 AC 444.
He was a member of the Insolvency Law Review Committee under the Chairmanship of Sir Kenneth Cork (1976 – 1982) which led to the passing of the Insolvency Act 1986; and continued to appear for the Department of Trade and Industry in numerous commercial cases.
While a QC he appeared regularly in Singapore and Hong Kong, and from time to time before the Federal Court of Appeal in Kuala Lumpur. He appeared frequently before the Privy Council in appeals from those jurisdictions.
Lord Millett was appointed a High Court Judge in 1986 and was promoted to the Court of Appeal in 1994. He decided several leading cases both at first instance and as an appellate judge. He tried the first civil action to result from the collapse of the Maxwell Empire. He also gave leading judgments on
Lord Millett was appointed a Lord of Appeal in Ordinary in 1998. As such he sat on the Appellate Committee of the House of Lords, which constitutes the final court of appeal in the United Kingdom, and the judicial committee of the Privy Council, which heard appeals from a number of Commonwealth countries, including New Zealand and the Caribbean territories. In 2000 he was appointed one of the Non-Permanent members of the Court of Final Appeal of Hong Kong, and he sits there for four weeks a year.
Lord Millett has sat on many important cases in the Appellate Committee of the House of Lords and on the Judicial Committee of the Privy Council. In 1999 he was a member of the 7-man Appellate Committee which decided that Senator Pinochet was not entitled to immunity from prosecution and could be extradited to Spain. He has delivered major judgments on restitution, contract law, insolvency, partnership, bills of lading, tracing the proceeds of fraud, floating charges and other forms of financial security, pension funds and other matters. As a member of the Court of Final Appeal of Hong Kong he has delivered several major judgments, including one on company schemes of arrangement and another on accounting for capitalisation of interest.
Lord Millett retired from his full time judicial duties in the House of Lords in January 2004, but continues to sit on the Appellate Committee from time to time and as before in the Court of Final Appeal of Hong Kong.