In third place in most opinion polls until a week ago, Fillon squeezed his former boss, Nicolas Sarkozy, out in the first round of the centre-right primary.
Now, if the polls are to be believed, he is set to face Marine Le Pen of the far-right National Front (FN) in the second round of the 2017 presidential election.
The prospect of a right versus far right decider led some left-wing voters to pay two euros for the privilege of voting for Juppé.
But not enough.
As his supporters celebrated victory on Sunday night, Fillon thanked voters who decided he represents the "French values to which they are attached" and declared "the left means failure, the far right bankruptcy".
So what are those values and what has Fillon promised to do if he is elected president of France?
Understated manner: Style - and the absence of criminal proceedings against him - seems to be the reason Fillon came from behind to push Sarkozy out of the running in the primaries. A practising Catholic, born in Le Mans and becoming an MP first in his home region and then in Paris, his victory is being hailed as " the triumph of anti-bling", in contrast to the former president's sometimes-brash lifestyle. During televised primary debates he kept his cool, at one point refusing an invitation to trash his rivals. But there is an iron fist within the velvet glove. François Copé, whose primary bid was a spectacular flop, found that out when the two men fought a bitter and long-drawn-out battle for the leadership of ther UMP (later to become the Republicans) in 2012. Sarkozy was on its receiving end during the primary campaign when Fillon reminded the world of the several legal inquiries faced by his former boss with the words, "Who could imagine General de Gaulle under investigation?"
Cutting public sector jobs: An admirer of the late British prime minister Margaret Thatcher, Fillon wants to administer some of her cost-cutting medicine to the French public sector. While promising to reduce unemployment, he has pledged to cut 500,000 of France's 5.4 million public service jobs by natural wastage over the next five years. Juppé, who proposed to cut 200-300,000, said that was going too far and would mean not hiring vital workers like nurses and police officers. Fillon says he will provide 12 billion euros extra for law enforcement and the rest of the work will get done because the 35-hour working week will be scrapped and civil servants will go back to working 39 hours, for 37 hours pay. He also wants working hours in the private sector to be negotiated at company level.
Cutting public spending: The jobs cull will be part of a drive to reduce public spending by 100 billion euros over five years. Government spending will be cut by a third, local authorities' by 20 percent and the rest will come from social security. The minimum retirement age will be raised from 62, Fillon's own age at the moment, to 65. National health insurance will be limited to "serious or chronic ailments", leaving other costs to be paid by private insurance.
Cutting taxes for the rich: Fillon hopes to axe France's wealth tax and cut corporate tax from 33 percent to 25 percent but raise the VAT sales tax by two points.
Islam, immigration, assimilation: Like any self-respecting French presidential hopeful, Fillon has published a book. His chosen subject: "Defeating Islamic totalitarianism." While Juppé warned that insisting that Islam is "incompatible with the republic" would lead to civil war, Fillon has said that France "has a problem linked to Islam". Muslims must "accept what all the others have accepted in the past ... that radicalism and provocation have no place here", he declared. Following this summer's row over the burkini, he backed a ban on the all-covering swimsuit and in 2012 he criticised "ancestral laws" on halal and kosher dietary obligations. Rejecting multiculturalism, he criticised education that teaches children to "be ashamed" of French colonialism, adding that France "is not guilty for having wanted to share its culture with the peoples of Africa, Asia and North America". Immigration should be kept to a "strict minimum" and migrants must assimilate, he believes.
The family, gay rights, abortion: Although he voted against the Socialists' gay marriage law, Fillon has said that it is not realistic to repeal it becaus "we're not going to demarry people". But he does want to make adoption by gay couples conditional on a judge's decision in the light of "the child's interests". He wants to reinstate family allowances for the well-off, a measure that was axed by the Socialist government, and opposes access to fertility treatment for single women and lesbian couples. He and his Welsh wife have five children and, as a Catholic, he is against abortion but says he will not change the law. His traditionalist stance has won him support of a number of hard-right websites and groups, including leaders of the Sens commun group born after the demonstrations against gay marriage and former Sarkozy adviser Patrick Buisson.
Russia and Vladimir Putin: During the primaries Russian President Vladimir Putin praised Fillon as a "very principled man". The two established good relations when they were both prime ministers and Fillon wants Europe to get closer to Russia, believing Moscow has been provoked by the EU's eastward expansion. He has advocated an alliance with Russia and Syrian President Bashar al-Assad agains the Islamic State armed group in Syria.