Andrew Cayley was Chief International Co-Prosecutor of the Extraordinary Chambers in the Courts of Cambodia (“ECCC”), the Khmer Rouge Tribunal, from 27 November 2009 to 16 September 2013. As International Co-Prosecutor he was responsible, together with the National Co-Prosecutor, for the overall supervision of the four cases before the court and overall management of the Office of the Co-Prosecutors inside the ECCC.
From 2007 to 2009 he was co-counsel on defence teams for The Prosecutor v. Charles Ghankay Taylor before the Special Court for Sierra Leone and also for Ivan Cermak in his case: The Prosecutor v. Gotovina, Cermak and Markac before the International Criminal Tribunal for the former Yugoslavia (“ICTY“).
From 2005 to 2007 he was Senior Prosecuting Counsel at the ICC where he supervised and provided legal direction on the Security Council referred ICC investigation into serious human rights violations in the Darfur region of Sudan. 
From 2001 to 2005 he was Senior Prosecuting Counsel at the ICTY where he supervised and provided legal direction in the investigation of General Ratko Mladic, commander of the Bosnian Serb army. He also supervised various other investigations relating to events in the former Yugoslavia and participated in consequent prosecutions. He had been prosecuting counsel at the ICTY from 1995 to 2001. 
From 1991 to 1998 he was an officer in Army Legal Services, acting both for the defence and for the prosecution. He was seconded by the British government to ICTY from 1995 to 1998.
He was appointed Queen’s Counsel in 2012.

He was appointed a Companion of the Order of St Michael and St George (CMG) for his services to international criminal law and human rights in the 2014 Queen's birthday honours list.