Cornwell was born in London in 1944. His father was a Canadian airman, and his mother was English, a member of the Womens 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 mothers 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 BBCs Nationwide and was promoted to become head of current affairs at BBC Northern Ireland. He then joined Thames Television as editor of Thames News. He relocated to the United States in 1980 after marrying an American. Unable to get a Green Card, he started writing novels, as this did not require a work permit.

As a child, Cornwell loved the novels of C.S. Forester, chronicling the adventures of fictional British naval officer Horatio Hornblower during the Napoleonic Wars, and was surprised to find that there were no such novels following Lord Wellingtons campaign on land. Motivated by the need to support himself in the U.S. through writing, Cornwell decided to write such a series. He named his chief protagonist Richard Sharpe, a rifleman involved in most major battles of the Peninsular War.

Cornwell wanted to start the series with the Siege of Badajoz but decided instead to start with a couple of warm-up novels. These were Sharpes Eagle and Sharpes Gold, both published in 1981. Sharpes Eagle was picked up by a publisher, and Cornwell got a three-book deal. He went on to tell the story of Badajoz in his third Sharpe novel Sharpes Company published in 1982.

Cornwell and wife Judy co-wrote a series of novels, published under the pseudonym Susannah Kells. These were A Crowning Mercy,