Wald Hidala, who vehemently denied planning the ousting of President Muawiya Wald Sidi Ahmad Taya was handed a five-year suspended jail term and ordered to pay $1,550.
"This verdict is not just and does not reflect the proceedings that took place in front of the court during this long trial and we are going to lodge an appeal with the supreme court", said Wald Hidallah.
Of the 14 others on trial for the past four weeks in the country, eight were given suspended prison terms of two to five years and fined.
Six others were acquitted and released.
Speaking to Aljazeera, following the sentencing, Ali Wald Sniba, spokesperson for Wald Hidala said: "it is an unfair sentence as it is based on wrong judgement".
"Wald Taya's regime is an unjust regime", Wald Sniba said. "However, we will appeal and continue fighting this regime to lift the injustice imposed on Mauritanian people and practice our political activities according the constitution," he added.
Wald Sniba added that "we don't blame the Chief Justice".
"Wald Taya himself has condemned us as criminals and the judge is only abiding by his rules due to the absence of justice in the country".
In a final plea of innocence, Wald Hidallah had warned the court Mauritania could descend into civil war if the president did not act to change the precarious situation among his mainly Muslim countrymen.
"As God is my witness, I declare that I did not conspire with anyone," said Wald Hidala, a former army colonel and associate of the president, who finished a distant second to Wald Taya in the ballot and was arrested on the eve of the 7 November election.
Mauritania "is in a serious state that could degenerate into civil war if nothing is done to find a solution"
Muhammad Khuna Wald Hidala
He was then released, only to be re-arrested immediately after the ballot.
Mauritania "is in a serious state that could degenerate into civil war if nothing is done to find a solution," Wald Hidala told the court earlier.
He added "this is meant as advice and not a threat."
Wald Hidala's son, Sidi Muhammad, who was amongst those issued a suspended sentence, said his father would appeal the presidential election results before the constitutional council. "The fight will continue," he said.