Bosco Ntaganda, known as the "Congolese Terminator," has been found guilty of war crimes and crimes against humanity.
The convictions include murder, rape and the use of child soldiers in warfare.
He was found guilty of all the 18 counts of war crimes and crimes against humanity he faced. The court case took place in the Hague, where the judgment was read out on Monday.
Head International Criminal Court (ICC) Judge Robert Fremr said Ntaganda was a "key leader" who ordered troops to "target and kill civilians" in the Ituri region of the Democratic Republic of Congo (DRC) in 2002 and 2003.
The convictions include responsibility for an atrocity in a Congolese village, where children were disemboweled or had their heads smashed in.
The ICC said 2,123 victims of Ntaganda, whoincluded ex-child soldiers recruited under his watch, had been consulted in the trial.
Ntaganda's lawyers argued in the defense that he himself was a victim, as he had been a child soldier. He denied all charges.
The charges are from 2002 and 2003, when Ntaganda was deputy chief of staff of the military wing of the Union of Congolese Patriots (UPC).
He was also found guilty of pillaging and of forcefully displacing people from their homes.
'A great victory'
The court's verdict has been welcomed but there were some reservations about whether it would have long-lasting consequences for peace in the region.
Jean Bosco Lalo, who coordinates civil society groups in Ituri, told DW that "justice continues to do its work for us [...] but the situation on the ground has grown worse, and one wonders what impact this judgement by the ICC will have on these problems." Lalo said it was more important to him that criminal acts were prevented than that they were punished.
A victims' representative, Cherine Luzaisu, said the verdict was "a great victory" for victims who had been waiting for justice since 2002, and that it was also an homage to those who had died since. But Luzaisu said the surviving victims now expected compensation for the harm they had suffered.
Who is Bosco Ntaganda?
Bosco Ntaganda was born in Rwanda in 1973 and fled the country's genocide to the DRC. The Rwandan government had targeted people from his ethnic group, the Tutsis, during the genocide in the 1990s.
He joined, and then became the deputy Chief of Staff of the UPC, in 2002 and 2003. The case in the ICC focused on his actions as a high-ranking member of the UPC, in which he was accused and convicted of ordering violence targeting Lendu community in the fertile Ituri region of the DRC.
Thomas Lubanga, who was convicted of war crimes in 2014, led the UPC at the time. The ICC first called for Ntaganda's arrest in 2006. Then-DRC President Joseph Kabila did not arrest him in the interest of peace in the region, he said.
Army leader, then rebel
Ntaganda later joined the Congolese state army and rose to the rank of general. In 2012, however, he left the army and formed the M23 rebel group which was based in the North-Kivu region of the DRC. He has faced allegations of ordering mass executions, rapes and the use of child soldiers as a leader of the M23 movement. Some 800,000 people were displaced in the fighting across the country.
Ntaganda handed himself into the US Embassy in Kigali, Rwanda, in 2013, after his M23 group split. He was the first person to ever surrender voluntarily to the ICC.
"We hope for a lot from the international courts," the spokesperson of the Lendu community, who Ntanganda persecuted, told DW earlier this year. "We hope that the courts give the victims their rights back, and that the courts compensate them for Ntaganda's crimes."
The court has not yet announced a sentence but it could send Ntaganda to prison for life.