SUPER-SUB James Morrison and Roman Bednar both scored in the dying seconds to snatch victory from the jaws of defeat against rock-bottom United.
Colchester, who had lost seven of their last eight games, stunned The Hawthorns by racing 2-0 up inside 16 minutes thanks to goals in the space of 120 seconds from Chris Coyne and Medy Elito.
The Baggies regrouped and were level by the break after Kevin Phillips and Chris Brunt netted in the 36th and 39th minutes respectively.
Albion poured forward at will after the break but missed chance after chance before being hit by a sucker punch when U's dangerman Kevin Lisbie poked home in the 76th minute.
United were on course to complete an unlikely double over the Baggies until Morrison fired home his sixth of the season in the 90th minute.
And Bednar sent the home hordes into delirium three minutes later when he ended Albion's three-game winless run by heading his 17th of the season.
The hosts started the day in fifth spot - four points off the automatic promotion places - while Colchester were marooned five points adrift at the bottom - some 11 points off safety.
Mowbray rang two changes, with Bednar and fit-again Brunt coming in for Ishmael Miller and Pele, who both dropped to the bench.
Morrison, back from an ankle injury, and Neil Clement, recalled from a month's loan spell at Hull, were also named among the subs.
Albion started like a house on fire and almost went in front after just 20 seconds.
U's right-back Phil Ifil slipped as he attempted to head Phillips' cross away and Brunt smashed the loose ball just over from 12 yards.
Colchester had to make a change after just five minutes when centre-half Pat Baldwin limped off and was replaced by Adam Virgo.
With United desperate for points, they weren't afraid to throw bodies forward and top scorer Lisbie wasn't far away with a header from John White's tenth-minute centre.
And the visitors stunned the home hordes by scoring twice in as many minutes.
In the 14th minute, Albion failed to deal with Dean Hammond's left-wing free-kick and Coyne spectacularly bicycle-kicked the loose ball home from six yards.
United doubled their lead from their next attack.
Dean Kiely could only palm Ifil's dangerous centre to the edge of the box where the unmarked Elito controlled before lashing an unstoppable shot into the top left-hand corner.
Shell-shocked Albion regrouped and kept Colchester penned inside their own half.
And Phillips was left cursing his luck in the 22nd minute when he saw a 25-yard rocket bounce off the left-hand post, onto beaten keeper Dean Gerken's back and out for a corner.
The Baggies had to wait a further 14 minutes of patient prodding and probing to pull a goal back.
Zoltan Gera fed Carl Hoefkens down the right and the over-lapping full-back's low centre was swept home by the unmarked Phillips at the far post - his 22nd of the season.
Albion now had the bit between their teeth and Gerken had to pull off a fine save to keep out Bostjan Cesar's towering header from Brunt's free-kick.
But it was 2-2 in the 39th minute.
Brunt powered across the edge of the U's box before lashing a 25-yard drive inside the far post, giving Gerken no chance.
The Baggies piled on the pressure during the closing stages of the half and Gerken had to produce a brilliant reflex save to deny Phillips after the Baggies top scorer had exchanged a great one-two with Gera.
Gerken was also well positioned to hold onto Robert Koren's 15-yard snapshot in the dying seconds.
Both sides made changes at the break. Mowbray replaced Cesar with Clement while Colchester brought on Kem Izzet for goalscorer Elito.
Albion put the U's' goal under siege again from the restart and only Ifil's acrobatic goalline clearance kept out Gera's looping header from Jonathan Greening's corner.
The loose ball fell to Clement who, with his first touch of the game, lashed just wide from 15 yards.
From another Greening flag-kick, Paul Robinson headed just wide.
Albion wasted their best chance to date in the 53rd minute when the unmarked Bednar miscued Gera's exquisite centre over from six yards
Moments later, Gera supplied further ammunition for Bednar from the right. This time, the giant Czech saw his close-range volley pushed away by Gerken.
There was a brief lull in the goalmouth action in the 56th minute when United boss Geraint Williams was sent to the stands for comments made to the fourth official.
Albion thought they'd won a penalty in the 62nd minute when Phillips appeared to be pushed to the ground by Coyne, but referee Graham Laws waved the protests away.
Colchester were defending much more resolutely this half and Mowbray made a change in the 71st minute, sacrificing right-back Hoefkens for a third striker in Miller.
But Albion fell behind again in the 76th minute - from Colchester's first effort on goal in the second half.
The Baggies failed to deal with a left-wing corner and Lisbie stabbed the ball home from close range.
For the first time in the game, Albion lost their way in possession and struggled to trouble the United goal - until the 90th minute.
Sub Morrison leveled the scores again when he collected Brunt's pass and rifled a 20-yard shot past Gerken.
And there was to be further drama.
In the 93rd minute, Brunt swung in a terrific left-wing cross and Bednar glanced it home to put Albion ahead for the first time. Cue scenes of unbridled joy.
Yet the Baggies still had to survive one more scare before the points were finally secure. Martin Albrechtsen saved Albion's bacon by blocking sub Scott Vernon's goalbound shot in the 94th minute.
ALBION (4-4-2): Kiely; Hoefkens (Miller 71), Albrechtsen, Cesar (Clement HT), Robinson; Gera, Koren (Morrison 76), Greening, Brunt; Bednar, Phillips. Subs not used: Kim, Pele.
COLCHESTER (4-4-2): Gerken; Ifil, Baldwin (Virgo 5), Coyne, White; Hammond, Jackson, Duguid, Elito (Izzet HT); Platt (Vernon 62), Lisbie. Subs not used: Cousins (gk), Balogh.
GOALS: ALBION - Phillips (36), Brunt (39), Morrison (90), Bednar (90+3). COLCHESTER - Coyne (14), Elito (16), Lisbie (76).
BOOKINGS: ALBION - Bednar (removing shirt 90+3). COLCHESTER - Coyne (foul 71).
REFEREE: G Laws (Tyne & Wear).
ATTENDANCE: 20,433.
ALBION STARMAN: Chris Brunt.
PIC: ROMAN PUTS GAME TO BED...Bednar wheels away in delight after heading Albion's dramatic late winner
