Katanga, who the ICC said was the "alleged" leader of the Patriotic Force of Resistance in Ituri, was found guilty of crimes committed in a 2003 attack on the village of Bogoro in Ituri province in eastern Democratic Republic of the Congo.
"The crimes of murder, attacking civilians, destroying property and pillaging were part of the common plan," for the 2003 assault, the court said in a statement Friday.
Known by the alias Simba, the DRC warlord was "beyond reasonable doubt" found to be an accessory to the deaths of at least 200 civilians from the Hema ethnic group. The majority of those killed in the attack were under the age of 18.
Amnesty International, which described the 2003 attack as "vicious," said justice was served in the case.
"The verdict will provide victims with a measure of some justice and we hope that this will spur the DRC to tackle other cases that have so far escaped justice," Stephanie Barbour, director of Amnesty International's Center for International Justice, said in a statement.
The ICC acquitted Katanga of five counts of rape as a weapon of war and for taking direct part in the hostilities.