Arnull studied a BA in Law at the University of Sussex and at the Institut d'études européennes [fr], Université libre de Bruxelles.
He later qualified as a Solicitor of the Senior Courts of England and Wales within a Magic Circle law firm.
[2] Arnull wrote The General Principles of EEC Law and the Individual in 1990, assessing the impact of the European Court of Justice.
In a review, Lewis outlined: "It must also be said that the discussion is certainly thorough and scholarly and Arnull makes thought-provoking observations on the case law.
"[3] Arnull provided a memorandum to the UK's House of Lords after being asked to comment on the new roles the European Court of Justice would play in the Treaty of Nice[4] Arnull contributed to Channel 4's FactCheck on the Lord Pearson's claim: "Most of our national law is now made in Brussels" on Sky (6 April 2010)[5] In 2017, Arnull published European Union law: a very short introduction, a book aimed at the general public to introduce the laws of the European Union, within the popular a very short introduction book series from Oxford University Press.