No fighting broke out Thursday as military officials searched for militants and looked through residents' homes for weapons, The Times of London reported.
The Iraqi government had control of the city by the day's end, the report said.
The Shia Mehdi Army took control of the city two years ago when British forces gave it to Iraq's military.
"We can't say Amara was entirely bad, there are good people here, poor people. But the city was controlled by the (Mehdi) Army, and these people are all backed by Iran," Capt. Hussein Ali said.
U.S. and Iraqi officials said no one resisted as they seized control of the city.