The actress and human rights activist was in The Hague, Netherlands on Wednesday when the court handed down the guilty verdicts against Thomas Lubanga.
"This is their day where these children will feel that there is no impunity for what happened to them and what they suffered. And it's a really beautiful step toward ending of child soldiers around the world," Jolie said.
Lubanga will be sentenced later this year. He faces a maximum of life in prison.
Human rights advocates said the guilty verdicts against Lubanga - the first judgment in the court's 10-year history - should stand as a clear deterrent to armies around the world not to conscript children.
The Associated Press contributed to this report.