Four fire engines and around 25 firefighters tackled a fire at a block of flats on Rowley Way in South Hampstead.
The ground floor of a flat was destroyed by the fire. One woman left the building before the Brigade arrived and was taken to hospital by London Ambulance Service.
Control Officers took the first of nine calls at 1456 and the incident was over for firefighters at 1815.
Crews from West Hampstead, Paddington, Euston attended the scene.
The fire is believed to have been accidental and caused by an electrical fault.