The storm had weakened slightly since Sunday morning but carried top winds of about 200 kph.
US President George W Bush has declared “a major disaster” in the three southern US states hit by Hurricane Dennis.
The declaration covers Florida, Alabama and Mississippi and makes the states immediately eligible for federal assistance in addition to state and local funds made available for recovery
But the White House was not ready to announce a specific amount of federal aid to the ravaged region.
The hurricane’s eyewall, the intense part of the storm around its centre, swept ashore just east of Pensacola in northwest Florida, the US National Hurricane Centre said.
After leaving 32 people dead in Cuba and Haiti, Dennis roared northward into the Gulf of Mexico with powerful winds and a three to 4.5-metre storm surge that could swamp towns.
As it came ashore, Dennis was downgraded to a Category 3 hurricane on the five-step Saffir-Simpson scale, a hurricane with winds of up to 208kph capable of causing serious damage.
But this still made it as strong as Hurricane Ivan, which killed 25 people, caused A$18.94 billion in damages and destroyed or damaged 13 oil drilling platforms in the Gulf in September.
Earlier in the day, Dennis was a stronger Category 4 storm.
“These are really dangerous storms and the devastation that could take place is something that we’ve already seen,” warned Florida Governor Jeb Bush.
By 0500 AEST, the centre of the storm was about 32km east-southeast of Pensacola, heading roughly north at about 27kph, the hurricane centre said.
The storm’s centre was expected to cross the coast and authorities in Florida, Alabama and Mississippi urged more than 1.2 million people in vulnerable low-lying areas to leave their homes and many had already heeded the warning.
In Pensacola, emergency officials told residents who decided to ride out the storm at home to write their names in waterproof ink across their chests in case they were killed and needed to be identified.
Energy companies pulled workers off oil rigs and shut down some crude and natural gas production in the Gulf of Mexico, where the United States gets a quarter of its oil and gas.