It found that Sibilla was also present during a torture session.
Prosecutors had requested the maximum penalty of 25 years in jail for both men.
"It's a vindication of the Argentine labor movement, which was a principal target of the dictatorship, with the complicity of the companies," said Tomas Ojea, a lawyer for the victims, after the sentence was read.
"The next step will be against the company itself," he added. "We are going to evaluate the steps to be taken to hold the company to account. It has to explain its actions."
The court heard testimony from dozens of people including Pedro Troiani and Carlos Propato, who were trade union leaders at Ford in Argentina when they were detained in the factory and tortured by the military junta.
They were held prisoner for two years and letters were sent to their families claiming they had been fired for failing to show up for work.
Sibilla and Muller are currently out under a former of conditional release that bars them from leaving the country, but the judge said they would have to serve out their sentences in prison once all appeals have been exhausted.
Survivors and relatives burst into applause and cheers when the verdict was given.
The court also sentenced a former general, Santiago Riveros - head of the secret Campo de Mayo detention center where the men were held - to 15 years in prison.
The case marked the first time executives of a multinational company had been put on trial for crimes committed during the dictatorship.
The company itself was not implicated, but prosecutors sought to demonstrate there was complicity with the dictatorship responsible for the deaths and disappearances of some 30,000 people, according to human rights organisations.