Six fire engines and around 40 firefighters tackled a flat fire on Lichfield Grove in Finchley.
Part of a flat on the lower-ground floor of the building was damaged by fire. One person left the property before the Brigade arrived. There were no reports of any injuries.
Station Commander Mick Palmer, who was at the scene, said: "The fire was producing heavy, black smoke and it was challenging for crews moving around the property.
"Crews worked swiftly to locate the fire and quickly brought it under control."
The Brigade was called at 1647 and the fire was under control by 1750. Fire crews from Finchley, Hendon, Hornsey and Kentish Town fire stations attended the scene.
The cause of the fire is believed to have been accidental and involved an electrical light switch.