Slynn was born on 17 February 1930 to John and Edith Slynn and educated at Sandbach School, Goldsmiths, University of London, and Trinity College, Cambridge. He was called to the bar at Gray's Inn in 1956 before moving to One Hare Court alongside Fisher, Neil, Parker, and Richard Southwell QC, becoming a bencher in 1970 and Treasurer in 1988. He served as Junior Counsel to the Ministry of Labour between 1967 and 1968. He was the First Junior Treasury Counsel (Common Law), or "Treasury Devil", from 1968 to 1974.

Lord Denning said about Slynn in his capacity as such: "He was outstanding. The best I have ever known. He will go far."^[1] His successful application to take silk in 1974 coincided with his becoming the first Leading Counsel to the Treasury.

He married Odile Marie Henriette Boutin in 1962.

He was appointed Recorder of Hereford in 1971 and as a judge of the Queen's Bench Division of the High Court in 1976, receiving the customary knighthood,^[2] serving additionally as President of the Employment Appeal Tribunal from 1978. In 1981, he left both these positions to become an Advocate General at the European Court of Justice (ECJ), and was appointed a Judge in 1988, a position he held until 1992.

He was appointed a Lord of Appeal in Ordinary on 11 March 1992, becoming a life peer as Baron Slynn of Hadley, of Eggington in the County of Bedfordshire,^[3] and being sworn of the Privy Council. He was a dissenter in the case R v. Brown, which upheld the legality of the criminal convictions resulting from Operation Spanner.^[4] As a member of the House of Lords, he served as Chairman of the House of Lords Select Sub-Committee on European Law and Institutions (1992–95), and as a member of the House of Lords Select Committee on Public Service (1996–98) and the Joint Parliamentary Committee on Corruption Bill (2003). He retired as a Law Lord in 2002.

He was appointed President of the Court of Appeal of the Solomon Islands in 2001^[5] and was life President of the Lord Slynn of Hadley European Law Foundation and President of the Civil Mediation Council. From 1992-1996 he was President of The Academy of Experts.

Slynn was a supporter of legal education. He wrote a foreword to the book, How to Moot: a Student Guide to Mooting^[6] and sat as a judge in the Central and East European Moot Court.^[7] He was Honorary President of the Durham Mooting Society and an honorary member of the Jefferson Literary and Debating Society at the University of Virginia. He was a patron of Staffordshire University's Law School.

Slynn was Patron of the UK wing of the Child in Need Institute (CINI) (CINI UK), founded by his wife Odile Slynn to help poor mothers and children in India.

He was a Trustee of The Loomba Trust, which cares for widows around the world, and Patron of the Bharatiya Vidya Bhavan Institute for Indian art and culture.

Slynn led a campaign to remove People's Mujahedin of Iran from the British and EU's blacklists.^[8]

Slynn received honorary degrees from numerous institutions, and was Visitor of Mansfield College, Oxford from 1995–2002 and of the University of Essex from 1995–2000. He was Chief Steward of Hereford between 1978–2008 and received the Freedom of the City in 1996, and was President of the Bentham Club in 1992 and of the Holdsworth Club in 1993. He was knighted in 1976. He was made a Knight of the Order of St John in 1998, having received the Order of St John in 1992, and received the Grande Croix de l'Ordre de Mérite (Luxembourg) in 1998; appointed a Knight Cross, Order of Merit (Poland) in 1999; Grand Cross, Order of Merit (Malta) in 2001; Officer's Cross, Order of Merit (Hungary) in 2002; and the Cross of Solomon Islands in 2007. He was appointed Knight Grand Cross of the Order of the British Empire (GBE) in the 2009 New Year Honours for his services to the International Law Association which he served as Chairman of the Executive Committee.^[9] In 2000 he was presented with a 2-volume Liber Amicorum: Vol I, entitled Judicial Review in European Union Law, was edited by Professor David O'Keeffe and Antonio Bavasso; Vol 2, entitled Judicial Review in International Perspective, was edited by Mads Andenas and Duncan Fairgrieve; both volumes were published by Kluwer Law International (ISBN 90-411 1373-8 (set)).

• List of members of the European Court of Justice

• Who's Who 2009

• Butler, Graham; Lazowski, Adam (2022). Shaping EU Law the British Way: UK Advocates General at the Court of Justice of the European Union. Oxford: Hart Publishing/Bloomsbury. ISBN 9781509950003.