Grays threw away a lead for the second game in a row as their Conference title challenge suffered a blow at Canvey.
The visitors had taken an early lead, Michael Knightly coverting a John Martin corner from close range.
But Canvey soon levelled, Jason Hallett firing home after Grays failed to clear a long free-kick from Ben Chenery.
Grays defender Jamie Stuart was sent off before the hour mark for violent conduct, with Dwayne Clarke firing home from 18 yards for the winner.
Canvey Island: Potter, Chenery, Duffy, McGhee, Sterling, Noto, Minton, Sedgemore, Keeling, Clarke (Ibe 67), Hallett (Gregory 90).
Subs Not Used: Jay, McKinney, Lowes.
Booked: Noto, Keeling, Gregory.
Goals: Hallett 15, Clarke 63.
Grays Athletic: Eyre, Sambrook, Nutter, Stuart, Angus, Kightly (Bruce 62), Thurgood, Olayinka (Hooper 75), Martin, Battersby (Oli 52), McLean.
Subs Not Used: Bayes, Slabber.
Sent Off: Stuart (60).
Booked: Kightly, McLean, Sambrook.
Goals: Kightly 4.
Ref: G Ward (Kent).