Eight fire engines and around 60 firefighters were called to a flat fire on Abbey Road in St John's Wood.
Part of a four-roomed flat on the fourth floor of the building was damaged by fire.
Station Commander Jonathon Holt, who was at the scene, said: "Firefighters worked quickly to bring the fire under control.
"One woman was taken to hospital by London Ambulance Service crews.
"There were road closures in place whilst firefighters worked to make the scene safe and buses were on diversion."
The fire is believed to have been caused by combustible items paced too close to an electric heater.
The Brigade was called at 1656 and the fire was under control by 1738. Fire crews from West Hampstead, North Kensington, Willesden, Paddington, Kentish Town and surrounding fire stations attended the scene.