But Hollande faces a tough challenge in introducing himself to a French electorate who know little of this political insider who was the party's chief bureaucrat and administrator and never held ministerial office.
He risks being defined by Martine Aubry, the leftist woman he defeated in the primary, who sneered at him as "a damp socialist" with few convictions and less passion.
Others in the party call him the key figure in "the socialism of adjustment," by which they mean that, like Tony Blair in Britain and Gerhard Schroeder in Germany, he recognized the limits of the state, the power of free markets and the need for profitable private enterprise.
His instinctive centrism, which may help him in the presidential election next May, helps explain the otherwise bizarre endorsement he won from the former President Jacques Chirac, a center-right Gaullist.
Hollande is also a passionate pro-European, who organized the Socialist Party's "yes" vote in the national referendum on the European Union's Lisbon Treaty. That vote signaled how far the main political parties and the French elite had lost touch with the public, who voted "no" in the referendum.
This could be Hollande's key weakness, since he has a reputation for being too intellectual and out-of-touch and, with the current crisis of the euro currency, the European cause is even less popular than it was.
Since Sarkozy is identified with the various (so far unavailing) attempts to rescue the euro and to hold Europe together with a close Franco-German alliance, another socialist candidate could have exploited the widespread anti-Europe sentiment. This is something Hollande cannot do.
Although he appeared Sunday night to have rallied the rest of the party behind him, Hollande's centrism means that he will probably lose a lot of votes to far-left parties in the first round of the presidential elections. This was what happened to the last Socialist prime minister, Lionel Jospin, when he was knocked into third place in the presidential election of 2002.
However, Sarkozy is equally threatened with losing many right-wing votes to the anti-immigrant Front National party.
Ironically, Hollande's father, a right-wing doctor in Normandy, who campaigned to keep Algeria French during the bitter colonial wars of the 1950s and early 1960s, would probably have voted for the Front National. Hollande learned his socialism as a student, but never with any of the fashionable far-left Trotskyite or Maoist groups. He was always a mainstream socialist and built a classic career of the French intellectual elite, going from the prestigious Science-Po school in Paris to the cradle of France's ruling elite, ENA (the Ecole nationale d'administration).
Founded by De Gaulle in 1945 to give France a world-class civil service, ENA has since given France two presidents and six prime ministers. Each year, some 1,500 of France's top students are allowed to sit a highly competitive exam for the 80 places. While at ENA, where his class included future Prime Minister Dominique de Villepin, Hollande met Segolene Royale, who was the Socialist party's losing presidential candidate against Sarkozy in 2007.
Hollande and Royale lived together for more than 20 years and they had four children together, all while working in politics, either on the presidential staff of Francois Mitterrand from 1981-95, or with the government of Lionel Jospin from 1997-2002.
But whereas Royale had high-profile ministerial jobs, Hollande was in the back room, trying to modernize the antique structure of the party, replacing the sclerotic grip of the declining labor unions with a mass membership.
His strategy bore fruit in the innovative primary system, under which any French citizen who wanted to participate could pay one euro ($1.30), declare support for the broad principles of the party (Liberty, Equality, Fraternity and social justice) and cast a vote. The result was that more than 2.5 million people gave legitimacy to the party and thus to Hollande's victory.
Separated from Royale since the aftermath of the 2002 elections, Hollande has put himself through a severe makeover He lost 25 pounds from his pudgy figure, developed a taste for sharp suits, expensive haircuts and designer spectacles. He has disciplined his formerly humorous and informal style of speaking into something more ponderous and supposedly "presidential."
But he hasn't lost his fondness for the sound of his own voice. Between his acceptance speech, his speech to his own supporters and another speech to party workers, he could be heard speaking for close to three hours non-stop on French TV Sunday night.
France is irritated by and bored with Sarkozy so Hollande is now the front-runner with a 20-point lead in the polls, which will certainly not last.
If he becomes president, it will likely be a case of Sarkozy losing rather than Hollande winning. But one thing seems certain, that French policy won't greatly change.
Ukraine in the balance
A poisoned chalice in Paris