The court considered it to be proven that such crimes occurred while Ntaganda headed the Patriotic Forces for the Liberation of Congo (FPLC) - the military wing of the Union of Congolese Patriots political group - in north-eastern Ituri province, between 2002 and 2003.
Ntaganda, dressed in a blue suit and shirt and wearing a red tie, stood motionless in the high-security courtroom as he listened through headphones while the judgement was read out.
Judges may chose to impose a life sentence instead, but in their ruling the judges said because numerous individual crimes involve overlapping conduct, they did not warrant a life sentence.
The public gallery was packed - not with friends, family and supporters, but students, journalists and human rights groups keen to witness Ntaganda go down in history as the first person to be convicted by the ICC of sexual crimes.
Sawyer, whose interview with HRW about Ntaganda was released after the sentencing, said she hoped the result would "carry the message to other warlords and serious human rights abusers that they understand they are not above the law, and even years after their crimes, they can be held to account".
The atrocities included a mass killing at a village where people - including children and babies - were "disembowelled or had their heads smashed in", the court said.More news: National Football League world reacts to latest Trent Williams drama
It has previously been accused of racism by African states because of what they say is its undue emphasis on the continent's crimes and misdemeanours. Ntaganda has already appealed against his conviction.
Nicknamed "Terminator", he was convicted on 18 counts, including murder, rape, sexual slavery and use of child soldiers.
The court ruled that the time since Ntaganda turned himself in, in March 2013, will be deducted from the sentence.
Ntaganda was a "key leader" who gave orders to "target and kill civilians", Judge Fremr said in the ruling.
An ICC spokesman confirmed it was the heaviest ever penalty handed down by the court, which was set up in 2002 to try the world's worst crimes.More news: U.S. inmate seeks release from jail after he 'died'
Ntaganda's UPC, dominated by the Hema clan, violently attempted to expel the Lendu people from DRC Congo's mineral-rich Ituri region in the country's far north-east.
Ntaganda's former FPLC commander Thomas Lubanga was sentenced to 14 years in jail in 2012.
A fifth person, Jean-Pierre Bemba, the former vice-president of DR Congo, was initially found guilty of war crimes before being cleared on appeal a year ago.
Ntaganda was also the first suspect to voluntarily surrender to the court, but judges were not convinced he'd done the honourable thing.
After the Ituri conflict, Ntaganda was integrated into the Congolese army and was a general from 2007 to 2012, but then became a founding member of the M23 rebel group in a new uprising against the government.More news: Silver Sluggers: Braves trio claim hitting honors for first time