Thomas Lubanga was found guilty in March of recruiting, kidnapping and abusing children in his Union of Congolese Patriots militia - sending them to kill and be killed during tribal fighting over land and resources in Congo's northeast Ituri region in 2002-2003.
Human rights activists working in Kinshasa hailed Tuesday's announcement.
Georges Kapiamba, a lawyer and president of the Congolese Coalition for the International Criminal Court, said the sentence "gives out a strong signal to all the other warlords and militias in North and South Kivu, but also in the eastern province, so they know that sooner or later they will be accountable, they will have to face international justice."
Lubanga's was the first guilty judgment passed down in the ICC's decade-long existence.
Tuesday's announcement was the first time the tribunal has sentenced a convicted war criminal.
Jonas Tshiombela Kabiena, the national coordinator of the network "New Congolese Civil Society", said the sentence was "strong message for the fight against impunity engaged in by the international community on our territory."
Prosecutors had asked for a 30-year sentence for Lubanga, but said they would be willing to cut it to 20 years if he offered a "genuine apology" to his victims.
Lubanga did not offer an apology.
Lawyer Irene Esambo, who is the president of the Centre for Studies in Justice and UN Resolution 1325 based in Kinshasa, said the sentence was not enough to compensate Lubanga's victims for what they had lost.
"What real compensation will these people see? This is symbolic compensation," she said.
ICC Judge Adrian Fulford said the time Lubanga has been detained since March 2006 will be deducted from the sentence, meaning the 51-year-old will be free before he turns 60.
Lubanga can appeal the sentence along with his conviction.
Lubanga's case this year has brought increasing pressure for the arrest of his much more infamous partner in crime, renegade Congolese army General Bosco Ntaganda.
Ntaganda had moved on from being a militia leader in Ituri to being the No. 2 leader in a tribal-based rebellion in 2006, when the ICC indicted both men for war crimes involving child soldiers.
Congo's back-to-back civil wars that drew in soldiers from a half dozen nations killed an estimated 5 (m) million people - more than any conflict since World War II.
- Democratic Republic of Congo