Cornwell was born in London in 1944. His father was Canadian airman William Oughtred and his mother was Englishwoman Dorothy Cornwell, a member of the Women's Auxiliary Air Force. He was adopted and brought up in Thundersley, Essex by the Wiggins family; they were members of the Peculiar People, a strict sect of pacifists who banned frivolity of all kinds, and even medicine up to 1930. Reacting to being raised by Christian Fundamentalists, he grew up rejecting all religions and became an atheist.