Armstrong was an engineer by occupation.[4] In 1940, he was arrested in Boston after being implicated in a Nazi spy ring. He was then deported to England. After returning, Armstrong was re-arrested and charged with treachery. He was tried on 8 May 1941 at the Central Criminal Court (the Old Bailey in London) and convicted for communicating with the German Consul in Boston, Massachusetts, to offer him assistance before the United States entered the Second World War. He was sentenced to death by Mr Justice Lewis.