Four fire engines and around 25 firefighters were called to a fire at an office block on Water Road, Wembley.
A small part of the ground floor was damaged by fire.
Willesden Watch Manager Simon Horn, who was at the scene, said: "Flames were coming out of the ground floor windows when we arrived and crews worked really hard to prevent the fire from spreading to the rest of the building."
The Brigade was called at 0022 and the fire was under control at 0118. Fire crews from Wembley, Willesden, and Park Royal fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.