In World War II, Vassall worked as a photographer for the Royal Air Force. After the war, he became a clerk at the Admiralty. In 1954, he was posted as Naval Attaché at the British embassy in Moscow. The year after he arrived, Vassall (who was homosexual) was encouraged by the KGB to become extremely drunk at a party, and was photographed in a compromising position with several men. The KGB used these photographs to blackmail Vassall into working for them as a spy. During his career, Vassall provided the Soviets with several thousand classified documents, including information on British radar, torpedoes, and anti-submarine equipment.
Vassall was identified as a potential spy when Anatoliy Golitsyn, a senior member of the KGB, defected to the United States. The KGB, worried that Vassall would be exposed, ordered him to cease operations until further notice. Another defector, Yuri Nosenko, added to the case against Vassall, but doubts about the evidence provided by both Golitsyn and Nosenko persisted. Vassall soon resumed his work. He worked as Private Secretary to Tam Galbraith, a junior Conservative Minister in the Admiralty. It became obvious to his work colleagues that Vassall had some other source of income for he moved to an expensive flat in Dolphin Square and threw lavish parties, but he explained that he had an inheritance from a distant relative.
On September 12, 1962, however, Vassall was arrested and charged with spying. He gave a full confession, but the documents which he admitted to stealing did not account for everything believed to have been taken. This led to speculation that there was another spy still operating in the Admiralty. Some have suggested that Vassall was deliberately sacrificed by the KGB in an attempt to protect the other (possibly more senior) spy. In October, Vassall was sentenced to 18 years in jail. A tribunal was held to inquire into whether the failure to detect him earlier amounted to a failure of intelligence, as many British newspapers had claimed; it exonerated the government.
Vassall was released in 1972, having served only ten years. He published an autobiography in 1975. In later life he changed his surname to Phillips, and worked quietly as a clerk at the British Records Association.