Colchester equalled their record victory even though they were forced to wait 39 minutes for their opener.
Greg Halford hammered home a free-kick and by the break it was two with Wayne Brown scoring from Halford's flick-on.
The floodgates opened on the resumption with Chris Iwelumo heading in before Jamie Cureton and Kevin Watson scored.
Mark Yates drilled home number six and set up Cureton with a cross before Jon Adams got a consolation for Leamington and Neil Danns scored a late brace.
Colchester: Davison, Stockley (Garcia 61), Chilvers, Brown, Duguid, Halford, Izzet (Danns 61), Watson, Yeates, Cureton, Iwelumo (Williams 73).
Subs Not Used: Baldwin, Gerken.
Goals: Halford 39, Brown 44, Iwelumo 48, Cureton 60, Watson 63, Yeates 67, Cureton 70, Danns 89, 90.
Leamington: Morris, Parisi (Smith 69), Tank, Gregory, Blake, Jon Adams, Morgan, Richard Adams, Rodman (Kelsall 64), Thompson (Eden 58), Howell.
Subs Not Used: Shepherd, Sidwell.
Booked: Gregory, Blake.
Goals: Richard Adams 72.
Ref: G Lewis (Cambridgeshire).