He faces corruption charges for allegedly using his influence to help his wife, Pojaman, purchase coveted public land in Bangkok at a reduced price.
If convicted, Thaksin and his wife could face more than ten years in prison, with no prospects for appeal.
Thailand's military accused Thaksin of widespread corruption when generals toppled his government in September 2006 while he was out of the country.
His allies won elections last December, paving the way for him to return from exile in February.
Despite more than a year of military rule, this is the first case against him to reach trial.
Earlier in the week, Thaksin was banned by the supreme court from leaving the country, after he requested to travel to China, Japan and the UK, where he owns property as well as the Manchester City football club.
Predecessors called
The hearing on Monday began with judges simply asking prosecutors to call the first witness: Banharn Silpa-archa, a former Thai prime minister.
Chuan Leekpai, Thaksin's immediate predecessor, was also expected to give testimony.
The case centres on whether Thaksin as prime minister had direct control over the central bank's Financial Institution Development Fund, which sold land to Pojaman at one-third of its estimated value.
Banharn and Chuan are expected to testify about what, if any, influence the prime minister has over the agency.
After Thaksin's case, the supreme court was set to decide on Tuesday whether to proceed with vote fraud charges against Yongyut Tiyapairat, a former speaker of parliament who has ties to the former prime minister.
Meanwhile, the constitutional court was deliberating on whether Noppadon Pattama, the foreign minister who was once Thaksin's personal lawyer, violated the law by signing a deal with Cambodia over a controversial temple on their border.