Lawyers for Mr Mladic, who is accused of genocide, opened his defence at the Yugoslav war crimes tribunal by calling the first of about 300 witnesses.
He has denied all accusations levelled at him by court prosecutors in The Hague. The trial has been delayed in the past due to his poor health.
Mr Mladic, 71, now frail and grey, began by refuting charges relating to the 1992-1996 Sarajevo siege, where a large portion of about 100,000 war deaths during the Balkan wars took place.
As many as 10,000 civilians were killed, sometimes while waiting in line for bread or collecting fire wood, by shelling and sniping in one of the longest urban sieges in modern history.
Mile Sladoje, 63, a former battalion commander, under Mr Mladic testified the Bosnian Serb forces "only defended" positions around the capital and never deliberately targeted civilians.
We "never received or gave orders to attack civilians" in Sarajevo, he told the court.
He countered that forces with the opposing army of Bosnia and Herzegovina held positions across Sarajevo, attacking Bosnian Serb positions from civilian sites.
"There wasn't a single area without military formation or military objects," Mr Sladoje said.
Mr Sladoje also denied his forces fired three shells in January 1992 that killed six children playing on the street.
Mr Mladic, was arrested in Serbia three years ago after 16 years on the run. Alongside Bosnian Serb wartime political leader Radovan Karadzic, he is accused of genocide for his role in atrocities, including the 1995 Srebrenica massacre of 8000 Muslim men and boys, considered Europe's worst atrocity since the Second World War.
Mr Mladic's indictment, first issued in 1995, alleges that between May 1992 and November 1995, he conducted a campaign of sniping and shelling against the civilian population "the primary purpose of which was to spread terror among the population".