Harris earned a First Class Honours degree in Modern History and a D. Phil. on French History at Exeter College, Oxford. He also won the Oxford University Gibbs Prize in History.
He was a special adviser at the Treasury (1981-1983), special adviser at the Home Office (1983-1985), director of the Conservative Research Department (1985-1989) and as a member of Prime Minister Margaret Thatcher's Downing Street Policy Unit (1989-1990).
After Mrs Thatcher left Downing Street, Harris continued to advise her and assisted her in writing a number of books.