Unknown attackers had snatched Margaret from a car while it was stuck in traffic on Thursday morning in Port Harcourt, in the southern oil-producing delta.
The abduction sparked outrage in Nigeria, including among armed groups in the Niger Delta who said it undermined their campaign for greater local control of oil revenues.
Emma Okah, a spokesman for Rivers State, said no ransom had been paid to secure the girl's release although Hill had said on Friday the kidnappers had called demanding money.
Okah said the release had taken place in the town of Ogbakiri in a rural area of Rivers State.
Security forces picked Margaret up after the kidnappers released her at an agreed location, but made no arrests.
The girl's mother said on Friday the kidnappers had called her, first demanding a swap between the daughter and the father – an oil worker – and later demanding a ransom, threatening to kill the girl.
Abductions for ransom are common in the Niger Delta, although it is rare for children to be targeted.
Margaret is believed to be the first foreign child abducted although two Nigerian children were snatched recently.