Spurs had entered the match unbeaten after three wins and a draw from their opening four league fixtures, but they were overwhelmed by a ruthless London City side led by former Ballon d’Or winner Alexia Putellas.
Putellas strikes early
London City needed only three minutes to break the deadlock.
Grace Geyoro's long-range effort was parried by Tottenham goalkeeper Lize Kop, but Putellas reacted quickest to fire home the rebound and give the visitors an early advantage.
The Lionesses continued to press Spurs and doubled their lead in the 22nd minute.
Lucía Corrales delivered a dangerous ball across the area, allowing Kadidiatou Diani to tap into an empty net.
Putellas makes it three
Tottenham struggled to cope with London's attacking movement and pressing.
Putellas struck again in the 34th minute, sweeping the ball home from a corner after Danielle van de Donk had helped create the opening.
Spurs went into the interval three goals down, despite Victoria Pelova and Julie Blakstad having opportunities to give the hosts some encouragement.
Van de Donk and Diani add to Spurs' misery
London City continued where they left off after the restart.
Danielle van de Donk made it 4-0 in the 52nd minute before Tottenham finally found a response.
Pelova pulled one back for Spurs in the 53rd minute, finishing from Julie Blakstad's cross to give the home side a brief lifeline.
However, any hopes of a comeback quickly disappeared.
Diani scored her second of the afternoon in the 74th minute after Tottenham goalkeeper Kop's clearance struck the French forward, allowing her to finish into an unguarded net.
Cascarino completes emphatic victory
London City completed the rout deep into stoppage time.
Delphine Cascarino found the net in the 90th+4 minute to make it 6-1 and underline the visitors' dominance.
The result ended Tottenham's unbeaten start and moved London City firmly into the early title race.
Match scorers
• Tottenham Hotspur: Victoria Pelova 53'
• London City Lionesses: Alexia Putellas 3', 34', Kadidiatou Diani 22', 74', Danielle van de Donk 52', Delphine Cascarino 90+4'
Tottenham Hotspur 1-6 London City Lionesses
Spurs face Chelsea next
Tottenham will now look to recover after the international break when they travel to face Chelsea at Stamford Bridge on 18 October.
For London City, the emphatic victory strengthens their credentials as a serious contender at the top end of the WSL, particularly after a performance in which Putellas and Diani both scored twice.