Around 40 firefighters were called to the scene in Wembley, where an area of around one square mile was left beneath flood water as deep as one metre.
Around 20mm of rain fell in some parts of the capital on Wednesday, forecasters said.
Photos taken in Wembley on Wednesday evening showed roads submerged, and parked cars stuck in around 10 inches of flood water.
The square-mile affected was in the area from Tokyngton Avenue to Stonebridge Road and Harrow Road.
One man was trapped inside a van near Stonebridge Station by the floods and had to be rescued by London Fire Brigade, while 19 other people were taken to safety.
Londoners had been warned on Wednesday to brace for a day of heavy rains and winds, with a “good chance” car and bus journeys would take longer because of standing water or hail, and “possible” delays to rail services.
That warning was lifted by Thursday, although rain was still expected.