Yingluck, whose elected government was toppled in a 2014 coup, fled the country last month after being charged over her government's failed rice policy.
The ex-prime minister-turned-fugitive was not present in court, which sentenced her to five years over subsidy scandal which normally attracts a maximum penalty of ten years in jail.
The search by the authorities for former premier Yingluck Shinawatra remains an ongoing investigative process, according to National Security Council's (NSC) Secretary-General Thawip Netniyom.
"The defendant was found guilty of the alleged offences under Section 157 of the Criminal Code and Section 123/1 of the Organic Act on Counter Corruption 1999 and was sentenced to five years' imprisonment", a statement from the court read. "Therefore, the action of the accused is considered negligence of duty".
The former leader was sentenced for failing to report false and corrupt government-to-government sales in a rice-pledging scheme.
She took office in 2011.
Dozens of supporters had gathered outside the court to hear the verdict on Wednesday.
The trail ran over two years, with a ruling initially scheduled for late August.
Photos posted on Instagram this week by one of Thaksin's daughters show Thaksin in London.
On the eve of the verdict junta chief Prayut Chan-O-Cha declared his "spies" had informed him of Yingluck's whereabouts, but said he would not reveal her location until after the judgement was delivered.