Buildings along the coast were destroyed by the "catastrophic" storm, one of the most powerful to ever hit the US.
Michael made landfall near Mexico Beach, a tourist town along the Panhandle, a 200-mile stretch of white-sand beach resorts, fishing towns and military bases.
The hurricane swamped streets, bent trees, stripped away branches and sent building debris flying across the beachfront.
"The window to evacuate has come to a close," said Federal Emergency Management Agency administrator Brock Long.
It began unexpectedly from a weekend tropical depression, becoming a furious Category 4 by early Wednesday, up from a Category 2 less than a day earlier.
The storm has the potential to drive sea water levels as high as 14ft above normal in some areas, the National Hurricane Center said.
It comes just hours after Florida governor Rick Scott warned locals it was "too late" to flee and he was "scared to death" people had ignored evacuation orders.
He said he hoped no one kept children with them as they chose to ride it out, but the time to evacuate the "target zone" had "come and gone".
"This is the worst storm that our Florida Panhandle has seen in a century," said Governor Scott. "Hurricane Michael is upon us, and now is the time to seek refuge."
"If you chose not to evacuate … you're not going to be able to get out. This thing is coming now. It's too late to get on the road," he told CNN.
Scott revealed communities across the Florida panhandle will see "unimaginable devastation," adding roof-shredding winds could easily top 150 mph.
Horror storm surges are also predicted leading to terrifying 31 ft high waves and devastating flash flooding.
About 3.8 million people are under hurricane warnings in Florida's Panhandle and Big Bend regions, along with parts of southeastern Alabama and southern Georgia.
Hurricane Michael what we know so far:
- The storm has intensified to a Category 4 major hurricane in the Gulf of Mexico
- It is expected to make landfall along Florida's northeastern Gulf Coast today
- More than 3.8m residents are under mandatory or voluntary evacuation orders
- Winds have already reached 150 mph and are tipped to blow even fiercer
- A Cat 4 or stronger hurricane has never made landfall in the Florida Panhandle
- Dangerous storm surge, damaging winds, 31ft waves and flash flooding are likely
- Hurricane and storm surge warnings have been issued along the Gulf Coast
- Heavy rain and strong winds will spread inland across parts after landfall
- President Donald Trump has declared a state of emergency for all of Florida
- Georgia Governor Nathan Deal has ordered 1,500 National Guards on standby
Tropical storm warnings cover 15.9 million people in several states.
The National Hurricane Centre described the hurricane – which has wind gusts near its centre topping 165 mph – as "extremely dangerous."
One meteorologist described the weather front as a monster as an apparition of a skull was spotted at the heart of Hurricane Michael in satellite images.
Only three major hurricanes Category 3 or higher have struck the Panhandle since 1950: Eloise in 1975, Opal in 1995 and Dennis in 2005.
Electric linemen shut off the power to Shell Point Beach prior to the arrival of Hurricane Michael
The area is a 200-mile stretch Florida lying between Alabama on the north west, Georgia on the north east and the Gulf of Mexico to the south
The National Weather Service in Tallahassee, Florida, warned: "A potentially catastrophic event is developing. Locations may be uninhabitable for weeks or months."
Federal Emergency Management Agency (FEMA) Director Brock Long says his agency is working "hand-in-hand" Governor Scott.
He praised Florida's use on Tuesday evening of the wireless emergency alert system to let residents know that the storm was getting stronger.
As for those who ignored evacuation orders, Long said people "who stick around and experience storm surge unfortunately don't usually live to tell about it".
Marco Rubio, a Republican Senator from Florida, said: "Every storm's different, but this storm is a monstrosity."
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