Thursday's verdict was the first in several legal cases brought against Thaksin, his family and associates by anti-graft investigators appointed after the September 2006 coup that ousted him from power.
Thaksin has long denied any wrongdoing by him or his family but the business tycoon suffered another setback on Wednesday when the Supreme Court agreed to hear new corruption charges against him.
The latest in a mounting string of legal challenges involves a controversial soft loan to Myanmar.
Accused of operating illegal state lottery in 2003.
Accused of giving illegal soft loans to Myanmar to benefit his family-owned telecoms firm.
Charged with helping his wife buy prime plot of Bangkok land for one-third of market value.
Accused of concealing assets in the names of family members and offshore companies.
Investigators accused Thaksin of a conflict of interest in a loan granted by the Export-Import Bank of Thailand so that Myanmar's military government could buy satellite services from his family-owned telecoms firm, ShinSat.
According to the allegations, Thaksin wrongly ordered the Exim Bank to increase a three-billion-baht ($89.6m) loan to four billion baht, so that Myanmar's ruling generals could buy more services.
ShinSat is part of the Shin Corp telecom firm, which Thaksin founded. His family sold the company to Singapore's state-linked Temasek Holdings in January 2006 in a tax-free deal that prompted street protests eventually leading to the military coup against him.
On Monday judges also agreed to consider charges against Thaksin and his aides stemming from a lottery scandal.
Thaksin and his then cabinet could be forced to pay back some of the $500m generated by the state lottery in 2003.
Thaksin's spokesman denies all charges, claiming they are attempts by his political opponents to discredit him.
"Thaksin decided at the beginning of the year to come back to Thailand because he was innocent and he's confident he can prove his innocence," Pongthep Thepkanchana told Al Jazeera.
Voters put Thaksin's allies back in government in December elections after two years of military rule.
But a pullout by the fourth-biggest component party this week, persistent street protests against Samak Sundaravej's government, and several forced resignations of ministers who are close Thaksin allies, have left the ruling coalition in disarray.