Ontario Provincial Police say the incident occurred as the girl and her family were preparing to leave an the Full Gospel Church in Frankford, a community north of Trenton, Ont., on Wednesday night.
The young girl was in the car in the church parking lot waiting for other members of her family when the vehicle reversed, crashed through a fence, went over the high retaining wall and plunged into the Trent River.
Onlookers shouted for help as the car was pushed a short distance downstream by the current before coming to rest on its roof, the rushing water filling the passenger compartment.
The pastor and his wife quickly jumped down into the water and tried to free the girl from the car.
"At first, because of locked doors, they weren't able to access the interior, but they were ultimately able to pull the freezing girl out through the rear hatch of the vehicle," Const. David Ludington said Thursday.
"Other churchgoers were able to lower a ladder over the edge and all three were pulled to safety."
The girl was taken to hospital by ambulance for treatment of hypothermia, but was later released.
The pastor and his wife were treated at the scene but did not require hospital treatment.
Ludington said it hasn't been determined why the car reversed into the river.