United scored three goals in nine second half minutes to set up a semi final with local rivals Manchester City in the Carabao Cup.
Ole Gunnar Solskjaer named a virtually full strength side for the visit of the League Two side and our place in the semi finals never looked in doubt despite a goalless first half. Last time the Reds got this far was in 2017 when we went on to win it under Jose Mourinho.
Marcus Rashford, as is his wont, missed a glut of chances but continued his fine run of form when he broke the deadlock early in the second half. He cut in from the left and thumped home a finish before Ryan Jackson's unfortunate own goal. Rashford then turned provider to set up strike partner Anthony Martial for the third goal.
Colchester offered little against their more illustrious opponents with Callum Harriott's strike from distance, an effort that flew narrowly wide, their best moment.
John McGreal's side - the lowest team left in the cup - defended deep and in numbers throughout and United were mainly restricted to efforts from long range. Rashford twice fired wide and had a free-kick saved by keeper Dean Gerken, who managed to avoid getting himself in a pickle.
Martial also went close with a 30-yarder and Juan Mata saw a snapshot held by the stretching Gerken. But the fourth tier side were punished six minutes after the interval as United sprung forward on the counter. The U's launched their first meaningful attack as Jackson's low cross was smothered by Sergio Romero. The Argentine keeper got down well to save from full-back Jackson and rolled the ball out to Nemanja Matic. He, in turn, swung it 40 yards into Rashford who galloped towards goal, cut inside Tom Eastman, outfoxed Luke Prosser and hammed home beyond the stranded Gerken - a 14th goal of his best ever season.
From then on, there was no way back even for a side who had got past United's divisional rivals Crystal Palace and Spurs on their way to a historic quarter final tie.
Five minutes further on, Mason Greenwood got free down the right and tried to pick out the onrushing Rashford, but a stretching Jackson could only turn into his own goal in his attempts to stop United's top scorer. Just after the hour mark, Rashford turned provider as he popped up at the far post to cut the ball back for Martial to stick out a foot and score. Nine minutes, three goals, game over.
Full marks to McGreal's gutsy team, with Prosser in particular epitomising their tenacious resistance. He threw himself in front of a Martial strike, and two long balls from Andreas Pereira caused problems, only for Rashford and Greenwood to fail to capitalise.
Substitute Jesse Lingard went close to a fourth but there was no further addition to the scoreline as United progressed to their first semi final under Solskjaer. We now face Pep Guardiola's men - two time defending champions in this cup - for a place at Wembley. The first leg will take place at Old Trafford in early January.
Overall team performance: 7/10
United Faithful Man of the Match: Marcus Rashford