Four fire engines and 21 firefighters and officers were called to a fire at a disused office block in Pentonville Road in Islington.
Part of the basement of the building was damaged by fire.
Two people left the building before the arrival of the Brigade. One man was treated at the scene for smoke inhalation by London Ambulance Service crews.
The Brigade was called at 1427 and the fire was under control at 1459. Fire crews from Islington, Stanmore, Shoreditch and Paddington fire stations attended the scene.
The cause of the fire is believed to be accidental.