Twelve fire engines and around 80 firefighters tackled a grass fire on The Avenue in Pinner.
A range of gardens and railway land was destroyed by the fire. There were no reports of any injuries.
The Brigade's 999 Control Officers took 16 calls to the blaze.
The Brigade was called at 1259 and the fire was under control 1514. Fire crews from Harrow, Northolt, Hillingdon, Heathrow, Heston and surround fire stations were at the scene as well as colleagues from Hertfordshire Fire and Rescue Service.