Ten fire engines and around 70 firefighters were scrambled to Brockley Hill in Stanmore on June 17.
Most of a disused two-storey hospital building, including its roof, was damaged by fire. There have been no reports of any injuries.
The brigade was called at 5.05pm and has been brought under control by 7.10pm but crews remained on site throughout the evening working to extinguish the fire completely, London Fire Brigade said.
A 32-metre turntable ladder was used at the scene and the Brigade’s drone team were also in attendance to assist crews.
Road closures were in place in the area while the incident was ongoing.
Crews from Stanmore, Mill Hill, Kentish Town, Finchley, Paddington and surrounding fire stations attended, along with Hertfordshire Fire & Rescue Service.
The cause of the fire is under investigation.
