Laurent Nuñez praised the "courage and determination" of firefighters who "risked their own lives" to salvage the building's stone structure and its two towers.
The fire ravaged the cathedral's roof and caused its spire to collapse.
French President Emmanuel Macron has vowed to rebuild it within five years.
The cause of the blaze is unclear.
"We now know it all came down to 15-30 minutes," Mr Nuñez said, adding that police and fire services would spend the next 48 hours assessing the security and safety of the structure.
Paris public prosecutor Rémy Heitz said his office was "favouring the theory of an accident", but had assigned 50 people to investigate the origin of the fire.
A number of companies and business tycoons have so far pledged about €800m ($902m; £692m) between them to help with reconstruction efforts, AFP reports.
Offers of help have also poured in from around the globe, with European Council President Donald Tusk calling on EU member states to rally round.
Eric Fischer, head of the foundation in charge of restoring the 1,000-year-old Strasbourg cathedral, told AFP the Notre-Dame may take "decades" to rebuild.