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 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 there were no such novels following Lord Wellington's 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 Sharpe's Eagle and Sharpe's Gold, both published in 1981. Sharpe's 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, Sharpe's 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, published in 1983, Fallen Angels in 1984, and Coat of Arms (aka The Aristocrats) in 1986. (Cornwell's strict Protestant upbringing informed the background of A Crowning Mercy, which took place during the English Civil War.) In 1987, he also published Redcoat, an American Revolutionary War novel set in Philadelphia during its 1777 occupation by the British.
After publishing eight books in his ongoing Sharpe series, Cornwell was approached by a production company interested in adapting them for television. The producers asked him to write a prequel to give them a starting point to the series. They also requested that the story feature a large role for Spanish characters to secure co-funding from Spain. The result was Sharpe’s Rifles, published in 1987, and a series of Sharpe television films staring Sean Bean.
A series of contemporary thrillers with sailing as a background and common themes followed: Wildtrack published in 1988, Sea Lord (aka Killer's Wake) in 1989, Crackdown in 1990, Stormchild in 1991, and Scoundrel, a political thriller, in 1992.
In June 2006, Cornwell was made an Officer of the Order of the British Empire in the Queen's 80th Birthday Honours List.
Cornwell's latest work, Azincourt, was released in the UK in October 2008. The protagonist is an archer who participates in the Battle of Agincourt, another devastating defeat suffered by the French in the Hundred Years War. However, Cornwell has stated that it will not be about Thomas of Hookton from The Grail Quest or any of his relatives.