Cazeneuve's government will work "on preparing the future until the end, until the last day", Hollande said while attending a trade fair after appointing the new prime minister.
The change led to a minor reshuffle, with Bruno Le Roux, who was in charge of the cabinet's relations in parliament, taking Cazeneuve's place at the Interior Ministry, former development and territorial reform minister André Vallini, a Hollande stalwart, taking Le Roux's old job and Jean-Marie Le Guen, a Valls ally, becoming minister of development and relations with the French-speaking world.
Cazeneuve's record as interior minister
With France still judged at risk from terror attacks, following those in Paris last year and Nice this year, the prime minister's principal task will be "to protect", Hollande said, and Cazeneuve's stint as interior minister was clearly his principal qualification so far as the head of state was concerned.
Born in northern France in 1963, Cazeneuve became an MP for a Normandy constituency in 1997 and has been minister for European affairs, the budget and the interior during the life of the current government.
As interior minister he has placed three principal challenges:
Terror attacks: Since the Charlie Hebdo attacks in January 2015, jihadists have killed 238 people in France. After tightening security and then introducing a state of emergency, the government claims to have foiled 17 terror plots since January 2016, carried out 4,000 house searches with 400 people placed under house arrest and about 20 mosques and prayer halls closed. Cazeneuve also oversaw the establishment of two bodies intended to improve the authorities' dialogue with Muslims in France.
Migration: The unprecedented influx of migrants from war zones and poor countries in the Middle East, Afghanistan and Africa has led to a Europe-wide political crisis, with France agreeing to accept 2,000 of the 30,000 the EU said it could take. The "Jungle" migrants' camp at Calais has sparked tension with the UK, where most of the migrants want to go, led to opposition in the town and aroused criticism by NGOs. The camp was dismantled in October and its 7,000 or so inhabitants sent to centres across France.
Police: Cazeneuve says he and his predecessor at the interior ministry, Manuel Valls, have kept Hollande's election campaign promise of recruiting 9,000 police officers and gendarmes, after a reduction of 13,000 under right-wing president Nicolas Sarkozy. That does not seem to have had a significant effect on the crime rate, nor has it satisfied the police force, with nationwide demonstrations demanding more resources after October's Molotov cocktail attack on two police cars near Paris. The government found 250 million euros to buy body armour and vehicles.
France's right-wing opposition dismissed the changes as "shuffling the pack", in the words of Republicans spokesperson Valérie Debord.
Far-right MP Gilbert Collard predicted that the new prime minister will go down in history as "Cazeneuve the Brief" with his five months in the post.
On the hard-left, Alexis Corbière, speaking for presidential candidate Jean-Luc Mélenchon, declared that they mean "continuity with what was already being done".
Valls launches campaign
Valls, whose campaign launch has been squeezed out of the headlines by his successor's nomination, on Monday evening promised to work for "reconciliation" if he is selected in the Socialist presidential primaries.
With polls predicting next year's deciding round will be between the mainstream right and the far right, he said that Republicans candidate François Fillon's predicted victory is "not set in stone" and that National Front candidate Marine Le Pen's policies would "ruin the working class".