Keith was born in 1949. He was educated at Thorpe Grammar School, Norfolk, the University of Hull, and undertook post-graduate research in War Studies at King's College, London. He is married to Pepita, daughter of Norman Hollingsworth. They have one son. As a military historian, Keith has been Director of Cranfield Security Studies Institute, Cranfield University, and a Senior Lecturer in War Studies and International Affairs at the Royal Military Academy, Sandhurst. He is author of five books on military history.
Keith was National Vice-Chairman of the Federation of Conservative Students from 1972 to 1973. He was Head of Foreign Affairs and Defence at Conservative Central Office from 1986 to 1988. From 1988 to 1990, he was Political Adviser to the Secretaries of State for Defence, George Younger, and Tom King. He was the Conservative Parliamentary candidate for Plymouth Devonport in 1992. At the 1997 General Election, Keith was elected as the Member of Parliament for Mid Norfolk and after boundary changes, he was elected for the new constituency of Broadland in 2010. In July 1997 he was appointed to the Conservative Parliamentary Defence Committee. In June 1998 he was appointed a frontbench defence spokesman. From June 1999 to June 2001, he was an Opposition Whip responsible for Treasury, Home Affairs and Health. He was subsequently appointed Shadow Agriculture Spokesman in October 2001. From 2002 to 2005, he was Shadow Minister for Defence and from May 2005 to May 2010 he served as Shadow Minister for Foreign Affairs.
Following the General Election in May 2010, Keith was appointed PPS to the Foreign Secretary Rt Hon William Hague MP.
In March 2015, he was appointed to the Privy Council of the United Kingdom and therefore granted the title The Right Honourable
Since 2016 Keith has been a member of the Intelligence and Security Committee.