Following his work as a teacher, Cornwell joined the BBC's Nationwide and was later promoted to head of current affairs at BBC Northern Ireland. He then joined Thames Television as editor of "Thames News". His first marriage ended in divorce in 1970s. He met his second wife in 1978 in Edinburgh while he worked for BBC Northern Ireland; she was a travel agent from the US and the mother of three children from a previous marriage. He relocated to the United States in 1979 after marrying her. He was unable to get a United States Permanent Resident Card (green card), so he started writing novels, as this did not require a work permit. He later became a United States citizen.