Percy was born and raised in New York City. She studied politics at Oberlin College in Ohio, later studying for a master's degree at the London School of Economics. She then became a researcher at the House of Commons where she spent six years; in her time there, she worked as a researcher for the MP John Mackintosh, who recommended her to the Granada Television producer Brian Lapping when he was looking for a researcher for a documentary on the workings of Parliament called The State of the Nation.
Percy produced the 1985 Granada series End of Empire, which explored the effects of the end of the British Empire in various former colonies, and worked with Lapping on the 1987 drama-documentary Breakthrough at Reykjavik, a reconstruction of the Reykjavík Summit between Ronald Reagan and Mikhail Gorbachev in 1986.
After fifteen years at Granada, Percy joined the newly formed production company Brian Lapping Associates in 1988. (When that company later merged with Brook Associates in 1997, she was a founding director of the new company Brook Lapping.)
The Percy-produced documentary series Watergate aired on the BBC and the Discovery Channel in 1994. Narrated by Daniel Schorr and directed by Mick Gold. This five-part series chronicled the Watergate scandal and featured exclusive interviews with many of the key participants in the events, including H. R. Haldeman, John Ehrlichman, John Dean and G. Gordon Liddy as well as former President Gerald Ford. The series won an Emmy Award.
The Death of Yugoslavia, with Lapping as co-producer, followed in 1995, which covered the events that led to the collapse of the former Yugoslavia and the aftermath. The series again contained interviews with many of the major participants, including Slobodan Milošević and Radovan Karadžić. The series won a BAFTA as Best Factual Series at the 1995 awards. The Balkans were revisited in the 2001 series The Fall of Milošević which dealt with the fall from power of the Serbian leader.
Percy, along with Brian Lapping, was awarded the Alan Clarke Award (for outstanding contribution to Television) at the 2002 BAFTA awards.
She was made a Fellow of the Royal Television Society in 1999 and was awarded the Judges Prize by that society at the 2010 RTS Awards. In 2009 at the Grierson Awards, she, along with colleagues from the Brook Lapping production company, won the Best Documentary Series award for Iran and the West; Percy also given the Trustee's Prize by the Grierson Trust for her contributions to documentary film over the previous 30 years. The Special Prize for Lifetime Achievement was given to her at the Orwell Prize ceremony in 2010.
In 2009, The Guardian wrote of Percy: " Her documentaries stand out for their seriousness, but most of all for the extraordinary range of people who agree to appear on them. These programmes do not depend on one celebrity autobiography, or a handful of journalistic talking heads; they interrogate players from all sides with a respect for complexities that demands concentration."
In 2004, she married the geneticist Steve Jones, having lived together since 1977.Inside Obama's White House (2016)
The Iraq War (2013)
Putin, Russia and the West (2012)
Iran and the West (2009)
Israel and the Arabs: Elusive Peace (2005)
Endgame in Ireland (2001)
The Fall of Milošević (2001)
The American Experience - Nixon's China Game episode producer (2001)
The Death of Yugoslavia (1995)
Countdown to War (1989)
End of Empire (1985)