Four fire engines and around 25 firefighters were called to a house fire on Rowley Way in St John's Wood.
The second floor of a four-storey mid-terraced house was destroyed by the blaze.
Station Commander Jon Singleton, who was at the scene, said: "Firefighters were faced with a well developed fire on the second floor of a mid-terraced property on arrival.
"Crews worked hard to stop the blaze spreading to adjoining properties.
"A man and woman left the property before the Brigade arrived and fortunately they were uninjured.
"Firefighters rescued a cat which has been taken to the vet."
The Brigade was called at 1355 and the fire was under control by 1453. Fire crews from West Hampstead, Paddington, Kentish Town and North Kensington fire stations were at the scene.
The cause of the fire is believed to have been electrical.