Ten fire engines and around 70 firefighters tackled a fire on Northway Crescent in Mill Hill.
A workshop with several vehicles inside, garages and gardens were destroyed by fire. There were no reports of any injuries.
Around 25 people were evacuated from surrounding houses as crews worked to bring the fire under control and cool gas cylinders present at the incident.
The fire was very visible and produced large amounts of smoke. Residents in the local area were advised to keep their doors and windows closed. Nearby roads, including the A1, A41 and M1, were extremely busy and members of the public were advised to avoid the area.
The Brigade's 999 Control Officers received 54 calls about the fire.
The Brigade was called at 1652 and the fire was under control by 2034. Crews from Mill Hill, Hendon, Stanmore and surrounding fire stations attended the scene. Firefighters from Hertfordshire Fire and Rescue Service also attended.
The cause of the fire has been recorded as undetermined.