Bangkok's criminal court handed out the sentence to the three election commission members on Tuesday, ruling they illegally acted in favour of candidates from prime minister Thaksin Shinawatra's Thai Rak Thai party.
The sentence strips them of their offices but the three vowed to file an appeal.
But, as members of the Supreme Court have previously urged the commissioners to resign on many occasions, it is thought such a move is unlikely to succeed.
The three men were denied bail, leaving them in prison at least for the night while their lawyers prepared to ask the Supreme Court to release them pending an appeal.
The charges against the commissioners came after elections held April 2, that were boycotted by opposition parties and then annulled.
Thai Rak Thai won the vote, but the victory was undermined by the fact that dozens of saeats were left empty due to the boycott.
The charges faulted the commissioners' organisation of re-runs of the elections to fill the empty seats. The court ruled that the handling of the re-runs illegally benefitted Thaksin's party.
"The facts in this case prove that the EC organised elections that were not free and fair and that violated the constitution," the verdict said.
"Both political parties and business leaders wanted to see the election commissioners resign"
Thanavath Phonvichai, political analyst
The constitutional court has already invalidated the election results, clearing the way for new polls on October 15 that were surprisingly approved by the country's revered king last week.
King Bhumibol Adulyadej ordered the courts to clean up the mess left by the election three months ago and is keen for a quick end to the resultant political turmoil has paralysed government decision-making and caused economists to trim economic growth forecasts.
The verdict represents a victory for Thaksin's rivals, who have accused the commission of bias that they warned would threaten the credibility of the new elections.
Thaworn Senniam, who filed the case for the opposition Democrat Party, said the ruling should ensure that the commissioners "cannot resume their work or continue their endless mistakes that have damaged our country."
There have been many calls for
the election commission to resign
Political analysts also welcomed the decision.
"Both political parties and business leaders wanted to see the election commissioners resign," said Thanavath Phonvichai, a professor at the University of the Thai Chamber of Commerce.
"If a new set of commissioners are appointed, that will minimise complaints about unfair elections."
A spokesman for the government, Surapong Suebwonglee, said the process to replace the commissioners would begin immediately.