An injury-time goal gave the Blades a first win in 2016.
The late goal was credited to David Edgar after a near post melee from a Martyn Woolford corner, in which Chris Basham was involved. Earlier Billy Sharp scored for the sixth consecutive league game.
Woolford and Coutts returned to the starting line-up, occupying the wide midfield roles, with Flynn and Baxter dropping to the bench. Former Blades striker Porter captained the home side.
On a cold day, the opening 15 minutes saw both sides make minor mistakes but they did not prove costly. For Colchester, Massey shot over and Sordell wide, whilst after Eastman's poor clearance, Done's effort was comfortable for goalkeeper Kean.
With the clock ticking around to 20 minutes, the Blades opened the scoring - with goalkeeper Long claiming an assist! His quick delivery saw Wynter fail to deal with the ball and SHARP twisted and turned before he affirmed his status as one of the top strikers in the league by tucking beyond Kean's outstretched right leg.
McEveley and Collins both went over to congratulate Long and in the following ten minutes both defenders made vital interventions as Colchester looked to drag themselves level. For the visitors, Done fired wide and Woolford had an effort blocked but as half-time approached all the Blades players in green were working tremendously hard to protect Long's goal.
Early in the second half, Done almost got the better of Eastman and a 50-yard run from Sharp put Colchester on the back foot, whilst the home side's best opportunity saw Hammond keep Massey away from goal. On 54 minutes, however, Colchester went very close but Long pushed around a post from Gilbey's 20-yard strike.
The effort, though, did kick off a spell of pressure which saw the Blades penned back. The central defenders did well to keep Porter away from a Sordell cross, but from the second phase from a set-piece on 59 minutes, EASTMAN glanced home a left-wing cross.
Soon after Long did well to hold a 30-yard effort from Sordell, whilst boss Adkins threw on Baxter and then Flynn in an attempt to wrestle back the initiative.
On 75 minutes, Baxter provided the pass that saw Sharp get between two defenders but he could only poke at the goalkeeper before he was surrounded by blue shirts.
The Colchester pressure was continuing, though, as mistakes presented them opportunities. After a combination of Long and Collins gave away possession, Hammond was forced to commit a foul which saw Moncur drill into the wall, whilst Long made an important save from Moncur after McEveley's header gave Sordell space.
The last ten minutes saw chances continue - at both ends. Two opportunities dropped to Baxter but he curled the first wide and then missed his kick from Hammond's cross. Long made another save from Moncur, whilst at the 90 minute mark, a melee of Sharp, Eastman and goalkeeper Kean saw the ball drop loose but was hacked clear.
The drama continued in three minutes of injury-time and included a Blades winner. Woolford's near post corner caused a commotion and the last touch was credited EDGAR, who touched in after Basham got the initial touch before the ball dropped into the back of the net at the end housing the Unitedites.
There was still time for a Colchester corner, which was hoofed clear, before the referee's whistle confirmed three very welcome points.
Colchester: Kean, Garvan, Briggs, Wynter, Gilbey, Porter, Moncur, Massey (Ambrose 85), Eastman, Brindley (Bonne 90), Sordell.
Unused: Edwards, Wright, Kent, Vincent-Young, Parish.
Blades: Long, Brayford, Basham, Coutts (Flynn 70), Sharp (Sammon 90), Done (Baxter 63), Collins, Woolford, McEveley, Edgar, Hammond.
Unused: Howard, Campbell-Ryce, Reed, Adams.