Ahead of the storm, officials in Florida had ordered a total of 6.3mn people, or about a third of the state's population, to evacuate.
More than 1,100 police, military officials and others were deployed to St Martin and the nearby French Caribbean territory of St Barts, where they used helicopters to identify the cars of people looting stores and homes.
Irma edged closer to Florida on Saturday after killing 24 people in the Caribbean islands earlier this week.
According to the Associated Press, some 30,000 people evacuated the Keys but "an untold number refused to leave, in part because to many storm-hardened residents, staying behind in the face of danger is a point of pride".
Those who did not evacuate are now on their own, as emergency responders will not venture out into the harsh winds of what is now a full-blown hurricane in southern Florida.
"Any time you're in that northeast quadrant as the storm is moving forward, that's where the maximum radius winds are that define the intensity of the storm".
"As soon as we start sending recon teams out the door, this is one of the first structures that needs to be checked", said Fire Chief Joseph Zahralban.
Thanks to widespread evacuations and the gradual weakening of the storm, Irma's wrath wasn't as devastating as it could have been.
The usually populous spot was all but abandoned as residents flee the coming storm.
Irma comes just days after Hurricane Harvey dumped record-setting rain in Texas, causing one of the costliest natural disasters in US history. The five feet of storm surge in Miami Beach and downtown Miami turned streets into rivers and made some areas impassable by car. "Storm surge is the deadliest hazard in a hurricane".
The weather service says the Smith Shoal Light station recorded a 119 km/h wind gust. She grows citrus fruits and raises cattle on the farm, which has been in her family for generations, she said.
At least 76,000 people were without power, he said, and the outages were expected to increase as Irma moved closer to the Florida coast. At 11 p.m. on Saturday night, it was extended as far west as the Ochlockonee River.
There have also been dozens of tornado warnings througout the state of Florida. Almost three months remain in the Atlantic hurricane season, which runs through November.
Although it's assumed planes and pilots would divert away from Florida, this image is still powerful.
He was providing updates from Sunday afternoon's emergency Cobra meeting which was chaired by Defence Secretary Sir Michael Fallon.