Fabius was François Hollande's main opponent during the referendum vote on the draft European Constitution in 2005. He will be assisted by Cazeneuve, the deputy mayor of Cherbourg who was named minister delegate for European Affairs.
Besides the Constitution, the two men voted against the Lisbon Treaty to reform EU institutions in 2007.
Fabius is a veteran of French politics. Elected first deputy for Seine-Maritime in 1978, he was minister for the Budget and Research and, at 38, became the youngest prime minister of the Fifth Republic - a post he held from 1986 to 1988.
'More united Europe'
A former president of the National Assembly, Fabius also had a brief stint in the European Parliament. In the government of Lionel Jospin, he managed the economy and finance in the transition from France to the euro.
During the referendum campaign on the draft European constitution, he sparked an internal crisis in the PS by refusing to follow the ‘yes’ party line, the position taken by Hollande. He claimed at the time that Europe needed a "more social, more human" Constitution and favoured the establishment of a "strong and united Europe".
That, however, did not prevent him from undertaking more liberal reforms during his various ministerial mandates, like the privatisation of highways or liberalisation of postal services.
In 2005, 55% of French people voted against the draft European Constitution. Less than a month later, the Netherlands followed the lead of France in rejecting the text by 61.5%.
After the success of the no vote, Fabius made an unsuccessful run for president in the 2007 Socialist primary.
Europe at the Elysée?
In the midst of the eurozone crisis, the election of a Socialist in the monetary union's second largest economy was seen by many analysts as a potential game-changer for the European Union. But Socialists defended Hollande's decision to name a foreign minister with ambiguous relations with Europe.
"Nobody doubts that Laurent Fabius is a statesman. An elected official of this magnitude will defend the interests of France and of Europe in the world. Do not caricature the 'no' position. Motivations that have led to this vote proved true," said the Socialist MEP Estelle Grelier, who is close to Fabius.
The two new ministers will not be the only ones to control EU affairs. The bulk of France's European policy is decided today at the Elysée, where Philippe Léglise-Costa, a pro-European who knows Brussels inside-out, will be secretary-general of European Affairs.
Léglise-Costa, who graduated from the elite École Politechnique, has juggled dossiers between Paris and Brussels throughout his career. In charge of external affairs for the representation of France in Brussels in 1995, he joined the cabinet in 1999 under then-Foreign Minister Hubert Védrine.
After a stint in New York, he worked as deputy director of Economic and Financial Affairs at the Quai d'Orsay in 2006. The following year he was appointed chief of staff by Jean-Pierre Jouyet, secretary of state for European Affairs. After recruiting new ministerial advisors, he left in 2008 to become number two at the Permanent Representation of France to Brussels.
He manoeuvred aggressively to find a compromise on the climate package advocated by France during the country's presidency of the EU in 2008. He got his way, as the agreement was hammered out after final negotiations with the Poles.
Alexis Dutertre, former advisor to Alain Juppé, will replace Léglise-Costa at the French representation in Brussels.