Germain Katanga showed no emotion as judges in The Hague convicted him as an accessory in the attack on the strategic village of Bogoro in 2003, in which some 200 civilians were hacked or shot to death and many women were raped and turned into sex slaves.
Katanga, nicknamed Simba by his men, which means lion, is only the second person convicted since the court was established in 2002.
Another alleged rebel leader originally charged with him, Mathieu Ngudjolo, was acquitted of all charges in December 2012.
In a 2-1 majority verdict, the court said Katanga played an important role in the attack on Bogoro by arming rebel fighters.
Presiding Judge Bruno Cotte said the defendant had a key position in "reinforcing the strike capability of the militia".
However one of the three judges hit out at the verdict, saying the court changed the nature of the charges against Katanga, depriving him of the ability to defend himself.
Katanga was originally charged as an "indirect co-perpetrator" in the crimes.
But judges said yesterday that they had changed the nature of his involvement to cast him as an "accessory," effectively watering down his involvement in the attack.
Defence lawyers were told of the possible switch months ago but Belgian Judge Christine Van den Wyngaert said in a written dissenting opinion that changing the charges "has rendered this trial unfair by infringing a series of Germain Katanga's rights".
Katanga will be sentenced after a separate hearing.
He is likely to appeal against the convictions.
The trial has been seen as a test of the ability of prosecutors at the ICC, which had handed down only one conviction and one acquittal in 12 years, to bring solid cases.