In 1985 he became the first British judge to write a book about a case he had presided over, the 1957 trial of suspected serial killer John Bodkin Adams.

[1] Devlin was involved in the debate about homosexuality in British law; in response to the Wolfenden report, he argued, contrary to H. L. A. Hart, that a common public morality should be upheld.

In 1948, Jowitt (by then Lord Chancellor) made Devlin, then aged 42, a High Court judge, assigned to the King's Bench Division; he received the customary knighthood later that year.

Amongst many commercial and criminal cases that Devlin tried, perhaps his most famous case was the 1957 trial of John Bodkin Adams, an Eastbourne doctor indicted for murdering two of his patients Edith Alice Morrell an elderly widow and Gertrude Hullett, a middle-aged woman whose husband had died four months before her death.

[8] Devlin directed the jury not to find for the prosecution unless they rejected all the defence arguments, and accepted this was a summing up for an acquittal.

Devlin later termed this "an abuse of process", done because the prosecution's case was deficient, and left Adams under the suspicion that there might have been some truth in talk of mass murder.

Devlin was initially extremely surprised because he had never heard of anyone accused of murder being granted bail, although he considered that Lord Goddard was not deterred by the lack of any precedent.

[11] In 1985, two years after the death of Adams, Devlin wrote an account of the trial, Easing the Passing – the first such book by a judge in British history.

Lord Hailsham told judge John Baker: "He ought never to have written it" before adding with a laugh, "But, it's a jolly good read".

[15] Lord Devlin received several honorary degrees, including from the universities of Oxford, Cambridge, Glasgow, Sussex, Leicester, Toronto, and Durham.

[16] After the Wolfenden report in 1957, Devlin argued, initially in his 1959 Maccabean Lecture in Jurisprudence at the British Academy,[17][18] in support of James Fitzjames Stephen that popular morality should be allowed to influence lawmaking, and that even private acts should be subject to legal sanction if they were held to be morally unacceptable by the "reasonable man", to preserve the moral fabric of society (Devlin's "reasonable man" was one who held commonly accepted views, not necessarily derived from reason as such).

H. L. A. Hart supported the report's opposing view (derived from John Stuart Mill) that the law had no business interfering with private acts that harmed nobody.

It is not possible to settle in advance exceptions to the general rule or to define inflexibly areas of morality into which the law is in no circumstances to be allowed to enter.

He noted that the chief of the "elastic principles" limiting the power of the state to legislate against immorality was "toleration of the maximum individual freedom that is consistent with the integrity of society".

[19][20] The American legal philosopher Joel Feinberg stated in 1987 that to a "modern" reader, Devlin's responses to Hart's arguments "seem feeble and perfunctory" and that most readers "will probably conclude that there is no salvaging Devlin's social disintegration thesis, his analogies to political subversion and treason, his conception of the nature of popular morality and how its deliverance is to be ascertained, or the skimpy place he allows to natural moral change".

[21] Devlin, for his part, considered (mainly in the last lecture in "The Enforcement of Morals") that the supporters of John Stuart Mill's doctrine had not plausibly fitted into their own theories such violations of the moral code as euthanasia, suicide, a suicide pact, duelling, abortion, incest, cruelty to animals, bigamy, bestiality and other obscenity, committed in private between consenting adults, causing no harm to others.

In 1959, soon after the declaration of the state of emergency in Nyasaland, the British Cabinet under Prime Minister Harold Macmillan decided to set up a Commission of Inquiry into the disturbances there and their policing, and appointed Devlin as chairman.