The Paris criminal court found Kunti Kamara guilty of crimes against civilians between 1993 and 1994, including a teacher whose heart he reportedly ate, and not preventing soldiers under his command from repeatedly raping two teenage girls.
The 47-year-old defendant betrayed little emotion when the verdict was pronounced.
The case was brought by the crimes against humanity division of the Paris court.
It was set up in 2012 to try suspected perpetrators of war crimes, crimes against humanity and genocide detained on French soil, irrespective of where their alleged crimes were committed.
The allegations against Kamara date back to the early years of the back-to-back conflicts that would ultimately kill 250,000 people between 1989 and 2003 in the West African nation.
The fighting was marked by mass murders, rape and mutilations, in many cases by child soldiers conscripted by warlords, with atrocities against civilians common.
Kamara was a regional commander of the United Liberation Movement of Liberia for Democracy (ULIMO), a rebel group that fought the National Patriotic Front of ex-president Charles Taylor.
The prosecution had sought a life sentence against Kamara, whose defence team had questioned the trustworthiness of the evidence against him.
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