Rory McAuley saw red as Lowestoft suffered a disappointing home defeat.
The Blues defender was sent off for his second yellow while Harlow's Jideofo Maduako was shown a straight red after a late challenge on Travis Cole sparked a melee.
The match was already lost for Lowestoft, who went behind on 36 minutes when Junior Dadson beat Ben Dudzinski with a fierce shot.
It was 2-0 on 54 minutes when Alex Read headed in a delightful ball from the right.
Lowestoft were having little joy against a strong Harlow backline and it was game over on 72 minutes when Jey Silva whipped in a fantastic ball in for Read to grab his second of the game.
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