The Hollywood film star co-founded The Sentry with US official John Prendergast - which has now blamed multi-national companies, greedy for oil, for funding fighting in South Sudan and profiting from it.
In a 64-page report, the organisation called on the US to expand its sanctions regime to target those involved and asked the European Union to impose sanctions on human rights violators and their networks.
It found that Dar Petroleum, the largest oil and gas producer in the country, provided direct support to deadly militias.
"And it has paid for government officials to live lavishly while the rest of the population suffers the consequences of a brutal civil war that has cost hundreds of thousands of lives," Clooney said.
South Sudan is the most oil-dependent country in the world, and Dar Petroleum is one of the country's most important entities.
It is comprised of China's state-owned China National Petroleum Corporation, Malaysia's state-owned Petronas, Chinese state-owned Sinopec, the private Egyptian-based firm SSTO and South Sudan's state-owned Nile Petroleum.
The Sentry - set up in 2015 - is formed of human rights lawyers, financial investigators, police officers and investigative journalists.
Clooney, who's also known for his advocacy in Sudan's western region of Darfur, unveiled the report at a press conference in London.
The report explores how a variety of international individuals and businesses are linked to armed conflict, corruption and atrocities during the country's five-year civil war that ended with a peace deal a year ago.
The oil consortium is the most prominent example.
People have been killed since 2013 in South Sudan.
While most of the country's oil rigs were shut down or destroyed during the war, the oil consortium continued operating.
Emails shown to the Associated Press by The Sentry show the South Sudan government directed Dar Petroleum to deliver drums of diesel to the "community Oil Protection Forces," armed groups with close links to the oil industry including members of the Padang militia, known for committing atrocities such as burning villages, targeting civilians and attacking the United Nations protection site in Malakal in 2016.