Elisa Baker, 43, of Hickory, N.C., also pleaded guilty to writing a fake ransom note before she and her husband, Adam, called police to report the girl missing last October, the Charlotte Observer reported.
The girl's remains were later found scattered around Caldwell County.
The guilty plea was given as part of an agreement between the defense and prosecutors. She had been charged with killing and dismembering the little girl, who had lost a leg to cancer.
Baker still faces federal drug charges, the report said.
Zahra Baker, who was born in Australia, moved to North Carolina with her father in 2008 after his marriage to Baker.