Brennan Dickenson scored twice as Colchester held off a spirited Grimsby fightback to win 3-2 at the Weston Homes Community Stadium in League Two.
Luke Prosser hit the bar with a header before Colchester took a 10th-minute lead when the captain headed the ball into the path of Dickenson, who swept home from inside the area.
Dickenson struck a post two minutes later and after Kayden Jackson curled just wide, Colchester doubled their lead 11 minutes before the break through Kurtis Guthrie, who seized on Sammie Szmodics' assist and fired emphatically past goalkeeper James McKeown.
Colchester made it 3-0 in the 39th minute when Dickenson decisively fired home his second from Drey Wright's low cross.
But Grimsby pulled a goal back three minutes after the break through half-time substitute Craig Disley, heading home Ashley Chambers' cross from close range.
McKeown denied Wright's diving header before Omar Bogle netted in stoppage time for Grimsby with a swerving shot to give the visitors hope but Craig Slater struck a post for Colchester, who held on for victory.