Hurricane Hermine hit the Gulf Coast of Florida early Friday, CNN reported, hours after the US National Hurricane Centre upgraded it from a tropical storm.
Packing winds of 120 kilometres per hour, the Category 1 Hurricane hit land in the Big Bend area on the north-western Florida coast.
The National Hurricane Centre said a "potentially life-threatening storm surge" was expected in the area, comments echoed by Governor Rick Scott.
Several counties issued mandatory evacuation notices for some Gulf Coast communities in low-lying areas, according to CNN.
The fourth hurricane recorded in the Atlantic this year, Hermine would be the first tropical storm to hit Florida as a hurricane in more than 10 years. Five people were killed when Hurricane Wilma made landfall in Florida in 2005.
Scott declared a state of emergency in 51 counties. At a news conference he warned of the dangers of possible tornadoes and fallen power lines.
Scott also ordered all state offices to close on Friday.
Utilities were down for 70,000 customers in Tallahassee, a city of around 181,000, the local government said on Twitter.
Crews scrambling to restore power were ordered to stop working due to safety concerns.
The weather agency issued a tropical storm warning for coastal locations extending from north-eastern Florida through Georgia as far north as Oregon Inlet in North Carolina.
Florida's Gulf Coast has seen sustained rain since Wednesday ahead of the storm.