Bosco Ntaganda, who maintained his innocence during his trial, faces a maximum life sentence following his convictions at the global court.
Bosco Ntaganda's defense say he was kidnapped and dragooned into military service as a boy, making him a victim of a brutal system.
Ntaganda, 46, was a "key leader" who gave orders to "target and kill civilians" judge Robert Fremr said in the ruling.
The judges also found Ntaganda guilty of crimes involving recruiting child soldiers, including young girls.
Ntaganda was the alleged Deputy Chief of Staff and commander of operations of the Union of Congolese Patriots (UPC), an organized armed group involved in two conflicts in Ituri in 2002 and 2003, according to the ICC.
"Men, females and children and infants have been found within the self-discipline".
Judge Fremr said 102 witnesses testified at Ntaganda's trial, including a woman who survived having her throat slit by Ntaganda's forces.
Later that year he became the first-ever suspect to voluntarily surrender to the ICC, when he walked into the United States embassy in the Rwandan capital Kigali and asked to be sent to the court, based in the Netherlands.
Bosco Ntaganda, who was known as "The Terminator", was convicted on all 18 counts, including murder, rape, sexual slavery and using child soldiers. "Some bodies were found naked, some had their hands tied up and some had their heads crushed. Several bodies were disemboweled or otherwise mutilated".
"And so in some ways, as one former child soldier told me, Ntaganda was a mercenary operating for these more powerful players", he said.
"The ICC and Congolese authorities should work together to bring to trial many more of those responsible for grave crimes, including senior officials", she added.
Maria Elena Vignoli, worldwide justice counsel with Human Rights Watch, welcomed Ntaganda's conviction. He fled the town as a teenager to Ngungu-Masisi in eastern DR Congo when the attacks on Tutsis started in Rwanda.
In 2012, he became the founding member of the M23 rebel group after he defected with a entire bunch of infantrymen from the Congolese nationwide navy.
Ntanganda took part in the rebellion that overthrew the Hutu-led Rwanda in 1994 and then went North to Congo, where he committed numerous massacres and other serious human rights abuses. As a minimal 800,000 other folks reportedly fled their houses.
General Ntaganda fought for various rebel groups that terrorised Ituri and North Kivu provinces over a period of almost 20 years.