The child was taken by helicopter to Johns Hopkins Children's Center in Baltimore, The Baltimore Sun reported. Doctors there were unable to save her feet.
The accident occurred Monday evening in Forest Hill, northeast of Baltimore. Rich Gardiner, a spokesman for the Harford County Volunteer Fire and EMS Association, said the father did not realize his daughter was in back of him when he changed into reverse while mowing the lawn.
The name of the family has not been released.
Johns Hopkins refused to comment on the specific case. But the hospital said lawn mower accidents are the most common reason for amputations in teenagers, and 95 percent of the lawn mower accident cases it treated from 2000 through 2005 involved amputations.