[4][5] Charles joined the Office of the Prosecutor (OTP) at the United Nations International Criminal Tribunal for Rwanda (UN-ICTR) as a legal adviser, from practice as a Solicitor in the United Kingdom, having been appointed by the then Chief Prosecutor of the UN-ICTY/UN-ICTR, Justice Louise Arbour (Judge of the Supreme Court of Canada and later UN High Commissioner for Human Rights) in January 1998.

His mandate was to lead teams of international criminal lawyers and investigators in the prosecution of persons who bore the greatest responsibility for the massacres in the Kibuye, Bisesero and Bugesera regions – the main Tutsi strongholds[6][7] during the 1994 Rwandan genocide.

Charles also led a team of international prosecutors in the retrial of Lt. Col Tharcisse Muvunyi, between 17 June and 2 October 2009, on the charges of direct and public incitement to commit genocide.

[24] However, on 29 August 2008, the Appeals Chamber reversed all the convictions against the defendant and ordered a retrial on the charge of direct and public incitement to commit genocide.

His work on behalf of the survivors and victims of the Rwandan genocide has been the subject of several books including Court of Remorse[35] by Thierry Cruvellier (Wisconsin Press 2010); Guilty Pleas in International Criminal Law[36] by Nancy Combs and Annotated Leading Cases of International Criminal Tribunals[37] by André Klip and Goran Sluiter (Intersentia 2005).

[40][41] He has also served as a visiting lecturer on international humanitarian law at the National Defence College, Nigeria and at the Nigerian Institute of Advanced Legal Studies.

[47] In 2013, while representing the outspoken Tanzanian opposition politician, the late Rev Christopher Mtikila, in a landmark and precedent-setting case against the United Republic of Tanzania before the African Court on Human and Peoples’ Rights, he successfully challenged the Tanzanian constitutional prohibition against independent candidature for election into public office as a violation of the African Charter on Human and People’s Rights.

[50] His advisory practice in international criminal and humanitarian law has included providing specialist advice to the Military High Command in Nigeria in connection with the preliminary examinations launched by the ICC into alleged crimes against humanity and/or war crimes committed in the context of armed conflict between “Boko Haram” and the Nigerian security forces.

[55][56][57] Since 2017, Charles has been retained by the Asset Management Corporation of Nigeria to investigate, trace and recover over 5 trillion Naira (14 billion US Dollars) currently owed to the Nigerian Government, following its acquisition in 2011, of non-performing loans from some of the country’s ailing commercial banks.