He returned on the same day a party allied to him secured enough parliamentary support to form a government.
Within hours of arriving at prison, however, Thaksin was transferred to a hospital, citing heart and chest-related ailments.
His sentence was later reduced to one year by royal clemency, and he spent six months in hospital before being granted parole.
Last year, the Supreme Court ordered Thaksin back to prison, ruling that his prolonged hospital stay was not fully justified and that the remainder of his sentence should be served in custody.
For two decades, Thaksin remained one of the most influential and divisive figures in Thai politics, shaping successive populist governments led by or aligned with the powerful Shinawatra family.
His legal troubles deepened after the collapse of the Pheu Thai-led government last year and the rise of former ally-turned-rival Anutin Charnvirakul to the premiership.