Sarkozy, 66, is unlikely to go to jail despite Thursday’s ruling.
He is expected to appeal the sentence, a move that will in effect suspend it.
The judge said he could serve the sentence at home with an electronic tag.
Sarkozy, France’s president from 2007 to 2012, was accused of having spent almost twice the maximum legal amount of 22.5 million euros ($26m) on a re-election bid that he lost to the Socialist Party’s Francois Hollande.
Part of the sum was allegedly used to fund extravagant campaign rallies and then hire public relations agency Bygmalion to mask the real cost of the events.
At Sarkozy’s five-week trial in May and June, the prosecution portrayed him as having a “cavalier” attitude to the public money available to candidates during campaigning, saying he ignored warnings from his accountants about the ballooning costs.
Al Jazeera’s Natacha Butler, reporting from Paris, said the court’s judge had concluded it was “clear that Sarkozy must have known that his campaign team were spending over the legal limit”.
The verdict was harsher than expected, she said, adding that with the latest conviction, Sarkozy’s political career was definitively “over”.
“Prosecutors had asked for a year-long sentence but with six months suspended,” she said.
“And in terms of the symbolism, it won’t be lost on the public that this verdict sends out a message to political candidates and parties that they cannot just act with impunity,” she said.
France is heading into a presidential election next year.
Sarkozy denies wrongdoing
The ruling came after Sarkozy was found guilty on March 1 of corruption and influence peddling in another case. He was given a year in prison, and a two-year suspended prison sentence in that case, but is free pending an appeal.
Sarkozy, who was not present in court on Thursday, has repeatedly denied wrongdoing in both cases.
He told the court in June that he had not been involved in the logistics of his campaign, nor aware of how money was spent during the election run-up.
“Can you imagine me going into a meeting to discuss the cost of flags?” he said, before claiming he had “too much to do”.
“From the moment I was told things were in order, I had no reason to give it more thought.”
But the court ultimately decided Sarkozy was made aware of the overspending, that he did not act on it and that it was not necessary for him to approve each individual expenditure to be responsible.