Luciano Becchio's goal just before the half-hour mark proved the difference in front of a bumper crowd of 9,559.
Jermaine Beckford headed wide early on for Leeds before goalkeeper Casper Ankergren made two impressive saves to deny Colchester's Karl Hawley and Mark Yeates.
Leeds then made the breakthrough through Becchio, the Argentinian striker slamming in his 16th goal of the season from the edge of the area after latching onto Robert Snodgrass' pass and evading challenges from Neal Trotman and Alan Maybury.
Simon Grayson's men pressed for a second but Becchio headed inches over and Us goalkeeper Mark Cousins denied Beckford just before half-time.
Colchester enjoyed plenty of possession in the second half but Beckford almost added a second for the Whites late on, only to be denied by Mark Cousins.