Mr Sarkozy spent nearly twice the legal limit on his failed bid for a second term in office.
The right-winger pulled out all the stops in 2012 to try fend off the ultimately victorious Socialist candidate, Francois Hollande.
A series of lavish United States-style election rallies caused his costs to spiral, with the final bill coming to at least 42.8 million euros ($AU69 million), nearly double the legal limit of 22.5 million euros ($AU36 million).
The case is known as the Bygmalion affair, after the name of the public relations firm which set up a system of fake invoices to mask the real cost of the events.
Mr Sarkozy, who remains a hugely popular and influential figure on the right despite being caught up in multiple investigations since losing office, was not in court for the verdict.
At his 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 and said he ignored warnings from his accountants about the ballooning costs.
Mr Sarkozy dismissed the allegations of wanton recklessness as "a fairy tale", saying he had been too busy running the country to pay attention to the finer details of his campaign finances.
He also denied any knowledge of the fake invoices.
Illegal campaign financing carries a maximum sentence of a year in prison and a fine of 3,750 euros ($AU6,000).
Prosecutors had asked the court to give Mr Sarkozy a one-year term, but to suspend six months of the sentence.
Thirteen other people, including Mr Sarkozy's former campaign manager, several Bygmalion executives and a handful of former directors of Mr Sarkozy's The Republicans party were also tried in the case. The court will deliver their verdicts on Thursday.
Corruption and influence peddling
In March, Mr Sarkozy became France's first post-war president to be handed a custodial sentence when he was given a three-year term, two years of which were suspended, for corruption and influence-peddling over attempts to secure favours from a judge.
Mr Sarkozy, who has accused the judiciary of hounding him since he lost his presidential immunity, appealed that verdict.
Before him, the only former leader to be sentenced at trial was Mr Sarkozy's predecessor Jacques Chirac, who received a two-year suspended sentence in 2011 for corruption over a fake jobs scandal relating to his time as Paris mayor.
Mr Sarkozy attended just one day of his campaign finance trial, a snub that infuriated prosecutors who accused him of acting "as if he is not answerable to the law like everyone else".
The case failed to garner much interest among the public, with the charges seen as less sensational than the corruption charges that had already dented any prospect of Mr Sarkozy making another comeback.
In 2016, he attempted to win back the Elysee Palace but failed to gain the nomination of his right-wing The Republicans party.
Mr Sarkozy was defeated by his former prime minister Francois Fillon, who was tipped to go on to win the election but crashed out in the first round over a fraud scandal that would later see him convicted.
Mr Fillon's downfall left the right rudderless and added to nostalgia among conservative voters for the heyday of the energetic Mr Sarkozy, who led France through the eurozone debt crisis of 2008-2009.
With new presidential elections looming in April next year, conservative candidates have been jostling to receive Mr Sarkozy's endorsement.