William Finlay was educated at Eton and Trinity College, Cambridge, where he placed third division of the third class in Part I of the classical tripos in 1897.

Criticising the appointment, the Law Times wrote:Of this gentleman's ability and qualifications for this important and not unrenumerative post we confess our entire ignorance—a lack of knowledge that is shared by the Profession generally.

The Law Times remarked that:Sir William Finlay ... must be accounted as a singularly fortunate man ... after but twenty-three years at the Bar, for no apparent Professional reason, he is passed over the heads of those who have undoubted prior claims for consideration and whose appointment would have strengthened the King's Bench.

On the bench, Finlay tried the revenue list, which he inherited from Mr Justice Rowlatt, and presided over a number of notable criminal trials.

His summing-up speech, which singled out the forensic evidence of Sir Bernard Spilsbury for the prosecution over those of several defence witnesses, proved to be controversial.

Norman Birkett KC convinced the jury to return a verdict of manslaughters by referring to the father's behaviour towards his family, and Finlay sentenced Meakin to a short term of twelve months' imprisonment.

[16] Finlay was promoted to be a Knight Grand Cross of the Order of the British Empire (GBE) in the 1945 New Year honours list.

[17] The same year, he was chosen as the British representative to the United Nations War Crimes Commission, in succession to Sir Cecil Hurst, whose health had broken down and who was frustrated at the British government's failure to respond to the Commission's proposals for the prosecution of war criminals.

Finlay was proposed as a candidate by Sir William Malkin, the Foreign Office Legal Adviser, who wanted to appoint a prominent figure to show that the British government attached importance to the Commission's work.

It was pointed out in Whitehall that Finlay "would be personally responsible for making the British machine work and could not therefore possibly accuse the Foreign Office or HMG generally for unnecessary delays, following the example of Sir Cecil Hurst.

[1] Finlay also complained that the British Army was so disorganised that it had allowed important evidence of war crimes to be lost.

[1] Finlay's health was affected by a trip to Buchenwald concentration camp with the UN War Crimes Commission in April 1945.

[1] Writing in the Oxford Dictionary of National Biography, G. R. Rubin described Finlay as "competent but not outstanding" both at the bar and on the bench, although he praised Finlay for his courage and zeal in his drive to make governments pursue German war criminals in the face of official obstruction and disinterest.

[1] They had one daughter, the Hon Rosalind Mary Finlay (1914–2002), who married Vice-Admiral Sir John Hayes in 1939.