Mr Sarkozy spent nearly double the legal limit on his failed bid for a second term.
The right winger did everything in his power in 2012 to try to push back the ultimately winning socialist candidate, François Hollande.
A series of lavish US-style election rallies have driven its costs up, with the final bill rising to at least € 42.8million (AU $ 69million), nearly double the limit legal value of 22.5 million euros (36 million Australian dollars).
The case is known as the Bygmalion Affair, named after the public relations firm that set up a system of fake invoices to hide the true cost of events.
Mr Sarkozy, who remains an extremely popular and influential right-wing figure despite having been caught up in multiple investigations since his loss of office, was not present in court for the verdict.
During his five-week trial in May and June, the prosecution portrayed him as having a “cavalier” attitude towards the public money available to candidates during the campaign and said he had ignored warnings from his accountants regarding balloon costs.
Mr Sarkozy dismissed the allegations of gratuitous 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 penalty of one year in prison and a fine of 3,750 euros (AUD 6,000).
Prosecutors had asked the court to grant Mr. Sarkozy a one-year sentence, but suspend six months of his sentence.
Thirteen other people, including the former campaign director of Mr. Sarkozy, several Bygmalion executives and a handful of former directors of Mr. Sarkozy’s Les Républicains party were also tried in this case. The court will deliver its verdict on Thursday.
Corruption and influence peddling
In March, Sarkozy became the first post-war French president to be sentenced to prison after being given a three-year sentence, including two years suspended, for corruption and trading in influence for attempt to obtain favors from a judge.
Mr. Sarkozy, who accuses the courts of harassing him since he lost his presidential immunity, appealed against this verdict.
Before him, the only former leader to have been sentenced at trial was Mr Sarkozy’s predecessor, Jacques Chirac, who received a two-year suspended prison sentence in 2011 for corruption in a bogus jobs scandal linked to his tenure at Paris city hall.
Mr Sarkozy only attended one day of his campaign finance trial, a snub that infuriated prosecutors who accused him of acting “as if he was not responsible to the law like everyone else “.
The case did not generate much public interest, the accusations being seen as less sensational than the corruption charges which had already dented any prospect of Mr Sarkozy’s return.
In 2016, he tried to win back the Elysee Palace but failed to secure the nomination of his right-wing Les Républicains party.
Mr Sarkozy was beaten by his former Prime Minister François Fillon, who was supposed to win the elections but crashed in the first round due to a fraud scandal which would later see him condemned.
Mr Fillon’s fall left the right rudderless and added to the nostalgia of conservative voters for the heyday of the energetic Mr Sarkozy, who drove France through the eurozone debt crisis of 2008-2009.
As new presidential elections loom in April next year, conservative candidates are scrambling to get Mr. Sarkozy’s endorsement.