Heidi Laub brought the child inside and called 911 around 10:45 a.m., according to the Lake County Sheriff's Office.
The child was wrapped in a black towel and still had placenta and an umbilical cord attached, Sheriff John Buncich said. The child was rushed to St. Anthony's Hospital. She was in good condition, suffering only a sunburn, Buncich said.
It was not immediately clear how long the baby was left in the yard.
"It's a miracle that the child is alive and is … in good condition," Buncich said. "We have nicknamed this baby our Miracle Baby Infant Jane Doe."
The full-term infant was described as Caucasian with brownish-blonde hair. Her age was undetermined, although the placenta and umbilical cord suggest she is a newborn.
"If you think about it, that 9-year-old child that went out to that yard to play, had she not gone into that yard to play, tragic results could've occurred," he said.
Authorities continue to investigate the case Wednesday. The child's mother remains unknown.
"There are laws in the state of Indiana that allow individuals that want to abandon a baby, they can do so … at a fire station, a police station, something like that, without any consequences," Buncich said.