The original intention of the authorities was to try Joyce for treason immediately, but when his complicated nationality issues came to light, the court case was forced back until September.
It was not clear that an individual, born in the United States, raised in Ireland, who obtained British citizenship via deception for a relatively short period, and who then obtained naturalised German citizenship, could legally be tried for treason against the British Crown, by broadcasting propaganda in an area outside of the Crown’s legal jurisdiction. Clearly, if Joyce owed no allegiance to the Crown, he could not be tried for committing treason against it.
When the case went to trial, on 17 September 1945, Joyce was charged on three counts of high treason, namely that:
1. William Joyce, on 18 September 1939, and on numerous other days between 18 September 1939 and 29 May 1945 did aid and assist the enemies of the King by broadcasting to the King's subjects propaganda on behalf of the King's enemies.
2. William Joyce, on 26 September 1940, did aid and comfort the King's enemies by purporting to be naturalised as a German citizen.
3. William Joyce, on 18 September 1939 and on numerous other days between 18 September 1939 and 2 July 1940 did aid and assist the enemies of the King by broadcasting to the King's subjects propaganda on behalf of the King's enemies.
The case took three days to hear. In order to overcome the confusion surrounding his citizenship, the prosecution argued that the British passport, which Joyce had renewed immediately prior to his escape from Britain, and which was valid until 2 July 1940, entitled him to the protection afforded to all British passport holders, and that he therefore owed allegiance in return. Had he kept his US citizenship during his time in Britain, he could never have been prosecuted on treason charges in the UK.
The judge, Mr. Justice Tucker, accepted the point of law presented, but instructed the jury to find him not guilty on the first two counts, which extended to periods beyond the validity dates of his British passport, when he was recognised in law to have been a US citizen.
The jury returned a verdict of guilty on the single remaining count of high treason. Carrying a mandatory capital sentence, Mr. Justice Tucker had no choice but to sentence William Joyce to death by hanging.
Despite general public satisfaction that Joyce had been brought to justice, there was widespread unease that his death penalty was as severe a sentence as those meted out to major war criminals, like those responsible for the massacres carried out in concentration camps. He had never directly claimed a single life, regardless of how reprehensible his political views might be.
On 27 September 1945, Joyce's lawyers gave notice of appeal, on the grounds that the Judge had ruled incorrectly that he could be expected to owe allegiance to the Crown during his time in Germany. The appeal was heard on 30 October and dismissed on 7 November.
Due to the important questions of law involved in the case, the Attorney General granted permission for the Joyce case to be heard before the House of Lords; the highest British court, which occurred between 10 and 13 December. The Lords also dismissed the appeal, on a vote of 3 to 1, on 18 December 1945.
All routes of appeal now exhausted, Joyce went to his death unrepentant and defiant saying: “In death as in life, I defy the Jews who caused this last war, and I defy the powers of darkness which they represent” according to the BBC.
He was hanged at Wandsworth Prison at 9am on 3 January 1946, the last person in British history to be hanged for treason. Like all executed prisoners, he was buried in unconsecrated ground within the prison grounds.