He was born in the United States to Irish parents during the years before Irish independence. However, Joyce, as his father did, identified himself as a Briton. The Joyce family left Ireland for London after the establishment of the Irish Free State. He attended St. Ignatius College from 1915 to 1921, including applying for university Officer Training Corps, and later studied at Birkbeck College of the London University. It was at Birkbeck that Joyce developed an interest in fascism. He was for a period a member of the Consertive Party.
Joyce was a prominent member of the British Union of Fascists under Sir Oswald Mosley during the 1930s. He was praised for his power of oratory. However, he left the BUF in 1937 and formed a breakaway organization, the National Socialist League. In late August 1939, shortly before World War II commenced, he and his wife, Margaret, fled to Germany, before British authorities could detain him under Defence Regulation 18b. Joyce became a naturalized German in 1940.
Besides broadcasting, Joyce's duties included propaganda among British prisoners of war, whom he tried to recruit into the British Free Corps, as a branch of the Waffen SS. He wrote a book, Twilight over England, that was promoted by the German Ministry of Propaganda.
Following the end of the war, he was tried as a British subject and executed for treason. There was a question of whether Joyce was legally liable for treason against Britain, as he technically never held British citizenship at all. However, Attorney General Sir Hartley Shawcross successfully argued that British law stated that anyone to whom Britain gave protection owed Britain loyalty.
Joyce was executed by famed hangman Albert Pierrepoint on January 3 1946, at Wandsworth Prison. The Crown considered trying his wife, Margaret, as well, but a secret memo recommended clemency for her.
William and Margaret Joyce had two daughters, Heather Landolo.