Judges found Martic openly sought to create a homogenous Serbian state encompassing part of Croatia and large parts of Bosnia, and deported tens of thousands of non-Serbs.
He was supported in this aim by Slobodan Milosevic, the former Yugoslav president.
Residents including women and the elderly were detained, beaten, tortured and murdered by special forces set up by Martic.
His forces also looted and destroyed properties to ensure non-Serbs had no homes to return to.
Martic led the opposition to Croatian independence from Yugoslavia in the largely Serb Krajina region.
Rebel Serbs seized control of the area effectively cutting Croatia in half, blocking transport links and ruining tourism.
Martic, a former policeman, was appointed president of a self-proclaimed Serb republic in Croatia in 1994.
He surrendered to the UN tribunal in 2002 and pleaded not guilty to all charges.
During the trial, which began in December 2005 and continued until January 2007, he said all he did was to protect the citizens of Serb Krajina regardless of where they were from.