Irma has killed one person in Georgia as rain flooded coastal communities.
Winds sent trees crashing onto homes and the world's busiest airport in Atlanta cancelled hundreds of flights as the storm's punch was felt statewide despite its weakened status as a tropical storm.
The city of Savannah, on Georgia's coast, was evacuated for the second time in less than a year because of the storm, and the National Weather Service in Peachtree City confirmed that Atlanta - more than 250 miles inland from either the Atlantic or Gulf coasts - was under a tropical storm warning for the first time.
One storm-related death had been confirmed in rural Worth County, Georgia Emergency Management Agency spokeswoman Catherine Howden said.
The storm fatality occurred in south-west Georgia, where Irma's centre crossed over from Florida on Monday afternoon. Tropical storm winds reached more than 400 miles from its centre, giving its powerful gusts and drenching rains a far reach.
Storm surge and rainfall arriving at high tide on Monday afternoon swamped communities along Georgia's 100-mile coast. On Tybee Island east of Savannah, Holland Zellers was heading home to grab a kayak so he could reach a home where his mother had taken shelter near the beach.
Shawn Gillen, Tybee Island's city manager, said waters appeared to be receding quickly but the flooding was extensive on the island of more than 3,000 residents.
"There's a lot of homes that have water in the them right now," Mr Gillen said.
The tidal surge also sent water and damaged boats rushing ashore into St Marys just north of the Georgia-Florida state line, said St Marys police Lieutenant Shannon Brock. Ms Brock said no injuries had been reported.
Almost all of Georgia was under a tropical storm warning.
A similar warning covered parts of South Carolina and most of eastern Alabama, where schools and businesses were closed. Alabama Emergency Management Agency meteorologist Jim Stefcovich said strong winds could linger in the state until 2am local time on Tuesday.
About 800 flights had been cancelled at Hartsfield-Jackson Atlanta International Airport, which remained operational on Monday as its staff monitored storm conditions with help from the Federal Aviation Administration, airport spokesman Andrew Gobeil said.
By Monday afternoon, more than 800,000 Georgia Power and EMC customers mostly in coastal and south Georgia were without power. Alabama Power said there were 12,000 outages mostly in the southeastern area of the state.
In Atlanta, falling trees and limbs may pose the most significant threat to life and property.
Amy Phuong, parks and recreation commissioner for the city, says six crews were already handling calls for felled trees around the city on Monday afternoon, as winds and rain began to intensify.
Ms Phuong says the crews expect to stay busy as Irma passes over the area and in the storm's aftermath.
About half the city's land area is covered by trees - a larger share than most urban centres.
Georgia's coast was largely empty after evacuations were ordered for the second time in less than a year. The coast's nearly 540,000 residents fled last October ahead of Hurricane Matthew, which caused an estimated 500 million US dollars in damage and killed three people.
The National Weather Service said flooding rains were a major concern on Monday, with 8 to 15 inches of rainfall predicted in south-east Georgia. Savannah saw winds strong enough to make palm trees bend and sway.
Further inland in Lowndes County near the Georgia-Florida line, firefighters rescued occupants of a few homes struck by falling trees, said county spokeswoman Paige Dukes. No serious injuries were reported. With wind gusts reaching 70 mph, officials ordered a daytime curfew for the 112,000 residents of Lowndes County, which includes Valdosta.
Metropolitan Atlanta Rapid Transit Authority suspended all bus and rail services on Monday and would decide later whether to resume operations on Tuesday, spokesman Erik Burton said.
Georgia Power spokeswoman Holly Crawford said the areas with the most power outages were coastal Glynn and Chatham counties. She says the utility company had about 3,400 employees on standby to respond, but cautioned repairs could take several days.