She suffered a severe head injury when the 6ft (1.82m) wall, which backs on to a public footpath, collapsed on Sunday evening.
The girl was airlifted from Fleetway, in Vange, Essex, in a "life-threatening condition", the East of England Ambulance Service said.
Essex Police said the scene had been cordoned off and the Health and Safety Executive had been notified.
Pictures from the scene show a child's bike which was damaged when the wall collapsed.
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Local residents told the BBC the injured girl was not riding the bicycle when the wall fell.
"Our officers are providing support to the girl's family at this difficult time," a police spokesman said.
A spokesman for Basildon District Council said: "While the incident appears to have taken place at the rear of a privately-owned property, the wall also stretches across the rear garden of a neighbouring council property.
"We are working hard to determine exactly what happened and how it happened."
A six-year-old girl was seriously hurt when a garden wall fell on top of her.