As he sought the nomination for the rightwing Republicans party last year, he talked up his reputation as "Mr Clean" -- in contrast to his rivals, former president Nicolas Sarkozy and ex-PM Alain Juppe.
Prospective presidents needed to be "irreproachable," he said, while ministers facing charges could not serve in his government "while being dogged by suspicion."
In late January, in an interview aired on national television, he declared that "there is only one thing that would stop me being a candidate: if my honour was called into question, if I was charged."
That is now a reality as Fillon was charged on Tuesday with misuse of public money and corporate assets over the employment of his wife as a parliamentary assistant for 15 years.
Though an MP employing a family member is not illegal in France, Penelope Fillon is accused of doing little for the 680,000 euros (725,000 dollars) she received in salary.
Fillon has denied all wrongdoing and says low-key Penelope worked with him doing constituency work from his home in the Sarthe region of northern France during his nearly four-decade political career.
The ex-prime minister has backtracked on his promise to withdraw his candidacy and is now running as an anti-system rebel determined to thwart the media, the government and magistrates who he says are working against him.
The transformation has left many former allies stunned.
"Some people hoped to bring me down, and bring down your voice with me, but you're here!" Fillon told cheering supporters in a now-typical display of defiance in the south of France last Thursday.
"It's you and only you who are the sovereign power, the guardians of democracy!" he said.
But the scandal has cost him credibility and support among his party and the electorate.
After being the clear frontrunner at the beginning of the year, polls currently suggest he would be eliminated in the first round of the election on April 23.
Low-key Penny
Silver-haired Penelope, from Wales and known as Penny by friends and family, was a university student in France when she met Fillon in their early twenties.
They soon married and live in an imposing manor house near Le Mans in northern France where they brought up their five children.
She was until recently a low-key political wife, a keen horse-rider who once described herself as a country "peasant" who preferred the countryside to the bright lights of Paris.
In examining Fillon's claims that his wife has "always" worked to help his career, the French media have also honed in on her past words.
"Until now, I have never got involved in my husband's political life," Penelope told regional newspaper Le Bien Public last year, echoing a similar statement to Britain's Sunday Telegraph newspaper in 2007.
'Mr Nobody'
After five years as prime minister under President Sarkozy from 2007-2012, Fillon emerged from his shadow during the campaign for the Republicans party's presidential nomination.
The long-time outsider accelerated past Sarkozy and former prime minister Alain Juppe in the final weeks, giving assured performances in a series of debates and opening up about his family.
In a widely watched television series, Fillon laughed about his "boring image" and talked about his private passion for mountains, racing cars and drones.
He won the rightwing nomination pledging radical economic reform for a country he said was "on the verge of revolt" after decades of chronic overspending, mass unemployment and a series of terror attacks since 2015.
Supporters also cheered his hardline stance on immigration.
He raised eyebrows when he told immigrants "when you enter someone else's house, you do not take over."
His outspokenness stood in contrast to his image as premier, of a quiet and urbane man whose steady temperament was in stark contrast to the impulsive Sarkozy.
Sarkozy once dismissed him as "Mr Nobody".
Once the youngest member of parliament at age 27, he is a devout Catholic who voted against gay marriage when it legalised in 2013.
The self-declared "Gaullist" -- a form of nationalism that proposes an independent and strong France -- also has a close bond with Russian President Vladimir Putin.
The two men overlapped as prime ministers from 2008-2012 and their closeness has led to questions about his foreign policy.
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