Cornwell was born in London in 1944. His father was a Canadian airman, and his mother, who was English, a member of the Women's Auxiliary Air Force. He was adopted and brought up in Essex by the Wiggins family, who were members of the Peculiar People, a strict Protestant sect who banned frivolity of all kinds and even medicine. After he left them, he changed his name to his birth mother's maiden name, Cornwell.

Cornwell was sent away to Monkton Combe School, attended the University of London, and after graduating, worked as a teacher. He attempted to enlist in the British armed services at least three times but was rejected on the grounds of myopia.

He then joined BBC's Nationwide and was promoted to become head of current affairs at BBC Nort