The court in northeast Mukdahan made its ruling based on photos showing the accused standing near or throwing burning tyres at the local government building in the province in May 2010, their lawyer Anon Numpa told AFP.
Sixteen other suspects were acquitted for lack of evidence.
Anon said he would appeal against the convictions and would ask for the 13 to be released in the meantime.
“We have applied for bail for all of them,” he said, adding that a ruling on the request was expected on Friday.
But Anon also said he considered the sentence was “moderate” considering that arson charges can carry the death penalty.
The Red Shirts held two months of protests in Bangkok last year that sparked a military crackdown which left more than 90 people dead, mostly civilians.
The violence also spread outside the capital, mostly to the Red Shirts’ stronghold in the northeast, including Mukdahan.
The Red Shirts were campaigning for early elections, accusing the then-government of being an unelected elite, because it came to power after a court ruling ousted allies of Thaksin, who lives overseas to avoid a jail term imposed in his absence for corruption.
Thaksin’s sister Yingluck Shinawatra is now prime minister after his party staged a comeback with a major victory in a general election in July.