[1] Ellis was a member of the UN-created Advisory Panel on Matters Relating to Defence Counsel of the Mechanism for International Criminal Tribunals.

[2] From 1999 to 2000, Ellis acted as Legal Advisor to the Independent International Commission on Kosovo, chaired by Justice Richard J. Goldstone, and was appointed by the Organization for Security and Co-operation in Europe to advise on the creation of Serbia's War Crimes Tribunal.

[2] He was involved with the trial of Saddam Hussein and also acted as legal advisor to the defense team of Nuon Chea at the Cambodian War Crimes Tribunal.

[4] Ellis also serves as Chair of the Management Board of the Central and Eastern European Law Initiative (CEELI) Institute.

From 1988 to 1990, Ellis worked in Washington, D.C. as an attorney at Klayman & Gurley, where he specialised in international trade matters, foreign direct investment and U.S. anti-dumping regulations.

The initiative was created after the fall of the Berlin Wall, with the purpose of providing international legal technical assistance to countries in Europe and Eurasia.

[4] When the International Criminal Tribunal for the Former Yugoslavia conducted its first trial, Ellis initiated and oversaw an assistance program for the defence team.

It reasoned that NATO had not been authorized by the UN Security Council, but the intervention was beneficial for the Kosovo population which was at a direct risk from the government crackdowns.

It is based in Johannesburg, and operates in Angola, Botswana, the Democratic Republic of Congo, Lesotho, Malawi, Mozambique, Namibia, Swaziland, Zambia and Zimbabwe.

For over twenty years, ILAC provided initial assessments of what is needed to rebuild workable justice systems in post-conflict countries.

[11] Moreover, Ellis initiated the creation of eyeWitness to Atrocities, a mobile phone application directed at using pictorial evidence of international crimes in a court of law.

It addresses evidentiary challenges to the use of photographic evidence by capturing metadata, including the hash values of photos, videos, and audio recordings.

[16] In February 2024, the BBC published a report detailing documented instances of Israeli soldiers abusing and humiliating Palestinian detainees, which Ellis said showed possible violations of international law regarding prisoners of war.

[18] He oversees the institute's activities in advancing the rule of law, including supporting and fostering its Central and East European Judicial Exchange Network, composed of judges from eighteen countries in the region.

"[31] In 2022, he was interviewed by Don Franzen, the Los Angeles Review of Books' legal affairs editor, on Russia’s invasion of Ukraine.

As part of this, a Ukrainian language version of the IBA-founded eyeWitness to Atrocities app was launched to capture potential evidence of war crimes.

[47] In April 2024, during talks with the Deputy Head of the Office of the President Iryna Mudra, Ellis voiced support for the establishment of a Special international tribunal for the crime of aggression against Ukraine.