TWO second-half goals in as many minutes from Gianni Zuiverloon and Simon Cox saw Albion deservedly progress into the Carling Cup fourth round.
The team sheet featured a staggering 21 changes from both sides' starting XIs at the weekend - but it was the Baggies' shadow side who excelled.
Only they will know how they trailed Jo's 19th-minute opener at the break, having bossed the half and created countless chances.
So it was no surprise when, ten minutes after the restart, Zuiverloon levelled the tie before Cox hit what proved the winner just two minutes later.
This was a notable scalp for the hosts against a City squad packed with multi-million pound talent - and the perfect way to follow-up Saturday's victory over Blues.
Roberto Di Matteo fielded a completely new team in the previous round at Leyton Orient. Tonight, he made one change less, with Nicky Shorey the sole survivor from the derby success.
Somen Tchoyi was granted his full Baggies debut on the right-hand side of midfield while fellow summer recruits Boaz Myhill and Pablo Ibanez were handed their competitive Hawthorns bows.
City boss Roberto Mancini named a totally different City XI to Sunday's victory at Wigan.
Albion made a bright start to this third-round tie and created the first shooting opportunity inside three minutes, although skipper-for-the-night Steven Reid's 25-yard rocket from Graham Dorrans' pull-back flew high into the Smethwick End.
Tchoyi went much closer three minutes later. After neat build-up play between Shorey and Roman Bednar, the Cameroon international powered past two defenders down the left before curling the ball just wide of the far post from a tight angle.
City threatened for the first time in the ninth minute when Jo was allowed to ghost onto Roque Santa Cruz's lay-off and whip in a low cross, which just evaded John Guidetti's outstretched boot in the six-yard box.
Dorrans was the next to try his luck but failed to trouble Shay Given's goal with a 20-yard free-kick.
However, the Scotland international was much more accurate with his next effort, forcing a fine near-post save from Given with a 25-yard exocet. The rebound flew straight to Bednar but his control let him down, allowing City to clear.
Albion continued to ask the questions, although Ibanez's downward header from Barnes' corner lacked the power to test Given.
But the Baggies' promising opening was undone when they conceded a soft opener on 19.
The hosts' defence failed to deal with City captain Patrick Vieira's punt forward and Jo swivelled and smashed a right-footed half volley past Myhill from the edge of the area.
Albion were soon back on the offensive and Dorrans did brilliantly to beat both Santa Cruz and left-back Greg Cunningham before seeing his acute drive blocked by Vieira in the box.
The former Arsenal star saved City's bacon again moments later when he got in the way of Tchoyi's fierce voley from the lively Barnes' far-post centre.
It was all Albion and Given had to pull off a fine one-handed reflex save to push away Bednar's hooked shot after a forceful Barnes run had created the chance.
The Baggies continued to keep City pegged back and, on 35, Given had to produce his best save yet to keep out Barnes' diving header from the impressive Tchoyi's centre. In the goalmouth melee that ensued, Bednar's shot was blocked by Ben Mee before Given denied Tchoyi - but the ball just wouldn't go in.
Albion really should have been level two minutes later when Cox and Dorrans scythed City open with a slick one-two. But Cox lost his composure in the box and, with only Given to beat, curled his 15-yard effort over.
The Baggies' profligacy continued three minutes before the break when Bednar, completely unmarked, failed to hit the target with a six-yard header from Dorrans' pinpoint cross.
As the half-time whistle blew, Albion left the field wandering how they were behind, having created numerous chances and limited City to a single effort at goal - the one they scored from.
Myhill was finally called into action again six minutes after the re-start when he did well to parry and then gather Guidetti's shot on the turn from Abdisalam Ibrahim's chip into the penalty area.
But Albion were soon back on the attack and Given proved their nemesis again when he rushed off his line to claw away Barnes' dink from Bednar's through-ball.
However, not even Given was able to prevent the Baggies turning the game on its head with two goals in as many minutes.
On 55, Zuiverloon muscled his way onto Bednar's lay-off before planting a clinical 15-yard finish into the far corner.
The equaliser was fully deserved - as was Albion's second on 57.
Bednar was again involved, flicking on Myhill's huge kick into Cox's path and the ex-Swindon striker beat Given all ends up with a right-footed finish from just inside the box.
It was turning into a cup cracker and Dedryck Boyata wasted a glorious chance to level when he headed Adam Johnson's corner over from six yards.
Back came Albion and Given had to be alert when Bednar deflected Tchoyi's wayward 15-yard drive goalwards.
The introduction of England star James Milner helped City finally get a grip of the game as the tie entered its final 15 minutes, although international team-mate Johnson was well off-target with a 25-yard drive.
Albion coped admirably until the second minute of stoppage time when Zuiverloon inexplicably conceded possession to Milner on the edge of his own box.
Myhill came to the right-back's rescue, brilliantly palming away the ex-Villa man's 15-yard effort to earn his side a fully-deserved passage into the competition's final 16.
ALBION (4-1-4-1): Myhill; Zuiverloon, Meite, Ibanez, Shorey (Jara 67); S Reid; Barnes, Cox, Dorrans, Tchoyi; Bednar (Fortune 82). Subs not used: Carson (gk), Olsson, Brunt, Odemwingie, Scharner.
CITY (4-4-1-1): Given; Vidal (Zabaleta 54), Boyata, Mee, Cunningham; Johnson, Vieira, Ibrahim (Milner 71), Jo; Guidetti; Santa Cruz (Silva 81). Subs not used: Taylor (gk), Kompany, Kay, Veseli.
GOALS: ALBION - Zuiverloon (55), Cox (57). CITY - Jo (19).
BOOKINGS: CITY - Vidal (foul 33), Vieira (foul 74).
REFEREE: N Swarbrick (Lancashire).
ALBION STARMAN: Somen Tchoyi.
PIC: CUP OF JOY...Cox is all smiles after hitting Albion's winner