Basildon Borough Council part-owned the boundary brick wall that stretched across the back of two properties in Vange, Essex.
The structure fell on the girl during a barbeque on 14 August 2016. She sustained life-threatening injuries and was put in an induced coma. She was in intensive care for seven days and in hospital for ten days in total.
An investigation by the Health and Safety Executive (HSE) found the council failed to act after private tenants raised concerns about the poor condition of the wall.
Fears about other walls that had fallen into decay were not passed to building control or the council's inspection team.
The HSE said the council had not implemented a system of intelligence-led inspection, maintenance and repair, and had failed to identify and remedy the risk of collapsing boundary walls, both owned solely by the council, or jointly with private residents.
Basildon Borough Council pleaded guilty to breaching s 3(1) of the Heath and Safety at Work Act and has been fined £133,333 and ordered to pay costs of £21,420.
After the hearing HSE inspector Tania van Rixtel said: "This was a wholly avoidable incident which could easily have been fatal. If Basildon Borough Council had properly recorded residents' concerns about the state of the walls, then a suitably qualified individual could have been engaged to identify the level of risk and instigated the required remedial action.
"Despite the low frequency of wall collapses, they are high consequence events requiring those with the responsibility for structural safety to take proactive measures to ensure that boundary walls and other structures are safely maintained."
