Jamie Cureton's last-minute winner for Colchester sent Leeds crashing back into the Championship relegation zone.
The visitors looked on course for a third-straight win when Eddie Lewis unleashed a left-footed drive inside the left post early in the second half.
But Colchester fought back through Chris Iwelumo, who headed home a Karl Duguid free-kick on 82 minutes.
Cureton then pounced to meet Hogan Ephraim's cross from the left to steer home a dramatic winner.
Colchester manager Geraint Williams:
"It was a hard-fought win and a bit of a scrappy game. But the lads kept going.
"The players are writing their own bit of history here and they are doing a great job."
Leeds manager Dennis Wise:
"People haven't done their jobs. We asked them to be in a certain place and they were not.
"They were poor goals to concede because we had looked comfortable.But we had a mad 10 or 12 minutes where we didn't defend properly."
Colchester: Gerken, Duguid, Baldwin, Brown, Barker, Garcia (Guy 69), Watson (Ephraim 58), Izzet, Jackson, Iwelumo, Cureton.
Subs Not Used: Davison, White, McLeod.
Goals: Iwelumo 82, Cureton 90.
Leeds: Ankergren, Rui Marques, Heath, Michalik, Gray, Douglas, Kishishev, Thompson, Lewis, Healy (Blake 89), Kandol.
Subs Not Used: Robbie Elliott, Foxe, Moore, Stack.
Booked: Ankergren, Kishishev, Lewis, Douglas.
Goals: Lewis 53.
Att: 5,916
Ref: A Wiley (Staffordshire).