Eight fire engines and around 60 firefighters were called to a flat fire on Oldfield Road in Harlesden.
Part of two flats on the ground and first floors of a terraced house converted into flats were damaged by fire. Part of the loft was also damaged by the blaze. One woman left the building before the Brigade arrived and was taken to hospital by London Ambulance Service crews.
Station Commander Peter Johnson, who was at the scene, said: "Crews carried out a search of the property and worked quickly to bring the fire under control.
"Firefighters remained on scene throughout the evening damping down and there were road closures in place causing traffic delays, so we asked people to avoid the area."
The Brigade was called at 1924 and the fire was under control by 2031. Fire crews from Willesden, Park Royal, North Kensington, Wembley and surrounding fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.