Hollande led the Socialist Party for 11 years and was leader when his partner Segolene Royale ran unsuccessfully for president against Nicolas Sarkozy in 2007. He emerged as the candidate after the downfall last May of Dominique Strauss-Kahn, who was then considered the Socialist favorite to defeat President Nicolas Sarkozy.
Strauss-Kahn was arrested after a New York hotel maid alleged that he tried to rape her. Charges against the former IMF chief were later dropped in the U.S. but he has been warned he could be investigated in France over accusations he participated in a prostitution ring.
One commentator said Hollande was maneuvering himself even before the scandal broke. "He's been preparing this campaign for 18 months now, much before DSK's demise," journalist Agnes Poirier told CNN.
"Some say that he knew, like actually many others in the party, that DSK was doomed: his colorful private life was always bound to prevent his running for president.
"In that respect, he's not a candidate by default," Poirier added. "He's simply a less charismatic personality than DSK, and less antagonistic than Sarkozy. It doesn't make him weaker though. If he wins, it'll be down to political skill, luck and the fact that Sarkozy is massively rejected by the French."
Born in 1954 in the northern city of Rouen, Hollande was the son of a doctor and a social worker. He was educated at the elite Ecole National d'Administration (ENA), where in 1978 he met Royal, and the couple started a three-decades relationship. They had four children together without marrying, before splitting a month after the 2007 election.
Hollande has represented the southern Correze region in parliament since 1988 but many question if he has the right stuff. The main obstacle to his election, analysts believe, is that despite being a Socialist Party insider Hollande has never formally held any national elective office.
The 57-year-old's electoral appeal is built around his affability, but the candidate continues to be dogged by questions from even within his own party about whether he has the charisma and decisiveness to be president.
Hollande himself said: "There's always a risk when the candidate becomes president: will he deliver what is expected of him?
"It's a choice, it's always an important moment for a country because it has to choose between two risks: either you keep the candidate who is on his way out or take the new candidate that we don't know. It's a gamble."
But after five years of Sarkozy's hyperactive premiership, during which time France's economic status has taken a knock, polls suggest voters are keen for a change from the president's flamboyancy. The bland Hollande was the early frontrunner in the campaign and opinion polls suggest he will beat the president by about 6% in the second round.
Before the first round, former president Jacques Chirac added his support to Hollande. Chirac's biographer Jean Luc Barre told French TV channel BFM TV: "He said last June that he will vote for Francois Hollande ... he has said it a few times since and then again 10 days ago."
Hollande is wary of complacency, saying half of those who declare they will vote for him will do so only because they are voting against Sarkozy.
"What the French want is coherence, stability and justice," Hollande said. "If I am in a favorable position today it's because my fellow citizens want to make the effort to straighten out the country, and at the same time they want it to be just and equitable, no one left out of national solidarity and no one left out of the contributions which must be made."
To his critics that sounds as if Hollande wants to revive left-wing tax and social policies of the past, a view reinforced in the first speech of his campaign when he attacked the financial community.
"I don't want to drive the markets crazy, I don't want to create trouble but rather order and rules and norms. We have to struggle against financial excesses ... those who speculate with sovereign debt, those who develop financial products which have done so much harm."
Given the constraints of international finance and economic structures, observers say that if Hollande is elected president he will not really have the room to maneuver to radically shift France to the left the way his Socialist predecessor Francois Mitterrand did three decades ago. What's more, they believe many voters may be making their choice for president this election based not on substance but on style.
And who can blame them, as Hollande has been criticized for declining to spell exactly what his policies will be on the economy, although he has pledged to increase taxes on the rich, boost social spending and create thousands of state jobs. He has also vowed to renegotiate the eurozone fiscal agreement, but analysts say Hollande will likely be a pragmatic leader.
The word one hears most often to describe Francois Hollande's style is "sympa" French slang for sympathetic, and his style could well transform his lead in the polls to a victory on May 6