Ontario Provincial Police Const. Larry Plummer says Brooklyn Honderich was found by a volunteer searcher just after 10 a.m. Monday, and was taken to hospital as a precaution.
The girl was found at the edge of a cornfield in a bean field, quite a distance from where she went missing the night before, he said.
Paramedics checked the girl over and she seemed to be "in very good health," Plummer added.
The girl's parents had reported her missing Sunday night after she wandered away from where the family keeps livestock in a barn near the village of Norwich, south of Woodstock.
"The parents were working on the farm and she basically wandered away when they were working," said Plummer.
A major search operation began immediately, involving members of several provincial police detachments along with emergency response and canine units who combed through area cornfields overnight.
More than 25 police officers were involved in the search effort, as well as numerous volunteers and a police helicopter, before the girl was found.
The amount of time she was missing was a concern, said Plummer, as was the weather.
"Thank God last night wasn't as cold as it has been," he said. "If it was 8 or 9 (degrees) Celsius, a very small girl, it could have been a problem."
But he called the outcome "a very happy ending."
Police thanked volunteers who joined in the search effort and Plummer estimated more than 60 had gathered at a local arena to help. He said the fact that a volunteer, not an officer, found the girl was not an issue.
"It doesn't matter — she was found," he said.
"It's just more important that she was found and thank God, she's safe and sound."
Note to readers: This is a corrected story; An earlier version said it was her family's farm.