The supreme court found her guilty of mishandling a rice scheme which led to at least $8bn in losses.
Ousted in 2014, weeks before a military coup, and later impeached, Ms Yingluck denies all charges and fled before the verdict, reportedly to Dubai.
Public opinion is divided. She remains popular with rural and poor voters.
During her trial, Ms Yingluck had argued she was not responsible for the day-to-day running of the scheme and insisted she was a victim of political persecution.
What rice scheme?
- Part of Ms Yingluck's election campaign, it was launched after she took office in 2011.
- To alleviate rural poverty, the government paid rural farmers nearly twice the market rate.
- The scheme hit Thailand's rice exports, leading to an estimated loss of at least $8bn.
Though popular with her rural voter base, the scheme was too expensive and open to corruption, her opponents said.