Slobodan Davidovic, a former member of the Serbian paramilitary unit known as "The Scorpions", was found guilty of participating in the killing of the six Muslim males from Bosnia's wartime enclave of Srebrenica.
"The accused actively participated in the whole operation from the moment he recieved an order from commander Slobodan Medic" to carry out the murders, said Judge Miroslav Sovanj.
Scorpions commander Medic is among five former Serb paramilitaries charged by Serbia's special war crimes court with murdering the six Muslims from Srebrenica in a case that emerged after a video of the shootings surfaced.
Their trial opened in Belgrade last week.
Davidovic's sentence included a nine-year term for torturing Croatian prisoners in the eastern village of Bobota, which was under the control of Serb rebels at the outbreak of the 1991-1995 Serbo-Croatian war.
The 52-year-old was arrested in the eastern Croatian village of Banovci in June this year after he was allegedly identified in the video footage.
The video was shown during the war crimes trial of former Serbian strongman Slobodan Milosevic, who is facing charges of genocide at The Hague for his part in the wars that tore apart former Yugoslavia.
The Scorpions unit, which was attached to Serbia's interior ministry, took part in the wars of the 1990s in Bosnia, Croatia and Kosovo.
The UN war crimes court has indicted several leading Bosnian Serbs for the murder of about 8,000 Muslim men and boys in the July 1995 Srebrenica massacre, the single worst atrocity in Europe since World War II.
The main suspects over the massacre are Bosnian Serb political leader Radovan Karadzic and his wartime general Ratko Mladic, who remain at large 10 years after their indictments were issued.