Pierce was educated at Penwortham Girls' High School and Girton College, Cambridge (BA in English, later raised to MA). She joined the Foreign and Commonwealth Office (FCO) in 1981. After a period of training in Japanese, she was posted to Tokyo as a Third Secretary in 1984. In 1987, she returned to London and joined the Security Policy Department of the FCO. From 1992 to 1995, she was based in Washington, D.C., as Private Secretary to the Ambassador. She held various positions at the FCO between 1996 and 2000: Team Leader for Ukraine, Belarus and Moldova (1996 to 1997), Deputy Head Eastern Adriatic (Balkans) Department (1997 to 1999), and Head of the FCO Newsroom (1999 to 2000).
From 2006 to 2009 Pierce was the UK's Deputy Permanent Representative to the United Nations in New York. In this capacity, she acted as the President of the UN Security Council in April 2007 and in May 2008. From 2009 to 2012 she was Director, South Asia and Afghanistan at the FCO, acting as the UK Special Representative for Afghanistan and Pakistan June 2010–June 2011. In 2012 she gained a MSc degree in International Strategy and Diplomacy from the London School of Economics. From 2012 to 2015 she was the Permanent Representative of the United Kingdom to the United Nations in Geneva, and also to other international organisations in Geneva including the World Trade Organization. From May 2015 to February 2016 she was the United Kingdom's Ambassador to the Islamic Republic of Afghanistan.
- PIERCE, Karen Elizabeth, (Mrs C. F. Roxburgh), Who's Who 2014, A & C Black, 2014; online edn, Oxford University Press, 2014
- Karen Pierce CMG, gov.uk
- "Birthdays". The Guardian. Guardian News & Media. 23 September 2014. p. 41.
- Change of Her Majesty’s Ambassador to Afghanistan, Foreign & Commonwealth Office, 31 December 2015
- "Change of UK's Permanent Representative to the United Nations and World Trade Organisation in Geneva". Foreign and Commonwealth Office. 5 August 2011. Retrieved 1 July 2012.
- New UK Ambassador to Afghanistan Presents Letter of Credence to President Ghani, Office of the President of Afghanistan, 26 May 2015
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