The airport is the world’s busiest, handling more than 250,000 passengers and almost 2,500 flights every day.
Passengers were left in darkened terminals or on board planes.
Many flights scheduled to arrive from other airports were being diverted elsewhere or held at their departure airport.
In a statement, the airport confirmed they had suffered a power outage shortly after 13:00 (18:00 GMT) on Sunday. Flights are expected to resume at 06:00 on Monday (11:00 GMT).
Georgia Power, which supplies the airport’s electricity, said it believed a fire at an underground electrical facility was to blame for the outage. Officials said a piece of its switchgear could have failed and started the fire, causing cable damage.