The court said that Thaksin, who is currently living in exile, had concealed shares in Shin Corp and geared several government telecom policies to favour the company.
Thailand froze $2.3bn of Thaksin's assets after he was forced from power and convicted of graft in absentia.
It was not immediately clear if the remaining assets, which the court found had been accumulated by Thaksin before he became prime minister, would be returned to him.
Speaking from Dubai in the United Arab Emirates, Thaksin, who is still widely popular in Thailand, dismissed the court's verdict as politically-motivated.
"This case is very political. The court was used to get rid of a politician," he said in a speech carried on one of the satellite television channels owned by his family.
"The ruling will be a joke for the world."
Al Jazeera's Wayne Hay, reporting from Bangkok, said that there was no course for Thaksin or others in his family to appeal against the verdict.
"This is the final court in Thailand, the supreme court," he said.
"There was talk that he would go to an international court, but it has been found that he has no right to do that because he does not have the support of the state of Thailand."
Thousands of troops and police were deployed across the country ahead of the ruling amid fears that it would trigger renewed protests by Thaksin's so-called red shirt supporters.
No unrest was reported after the ruling on what local media had called "Judgement Day".
But the pro-Thaksin movement, known formally as the United Front for Democracy against Dictatorship, did pledge to go ahead with a "million man march" in Bangkok on March 14.
"Our fight for democracy will continue. We choose to rally in order that people may digest the ruling and see whether it is fair to Thaksin," Jatuporn Prompan, a Red Shirt leader, said.
The release of the verdict was timed to minimise any impact to Thailand's stock market, which will close for the long weekend before the announcement, and not resume until Tuesday.
The political crisis triggered by Thaksin's ouster has left Thailand and its economy in a state of unrest and uncertainty over the past four years, with frequent protests, airport blockades and three changes in government in 15 months.