A quite remarkable finish to a tie much tighter than the scoreline suggests sent City surging through to the last 16 of the Europa League as the bemused holders reeled out at the Etihad Stadium.
Porto fell behind in just 18 seconds to a strike from man of the match Sergio Aguero, but they more than held their own thereafter until the Blues produced a grandstand finish and three more goals in the last 15 minutes.
Substitutes Edin Dzeko and David Pizarro both weighed in with Porto down to ten men. It meant an embarrassing 6-1 aggregate exit for the pride of Portugal, but it would be a harsh critic who claimed they deserved such a humiliation.
The quickest goal of the season - or many other seasons, for that matter - was a delight for City fans yet far from fazing the Portuguese, already 2-1 down from the first game, it seemed to strengthen their resolve.
But Porto, suddenly 3-1 down with the clock barely started on the second leg, shrugged it off as a minor blip and replied with some smashing one-touch football and spells of possession.
For all their admirable approach work, however, Porto lacked a real cutting edge
City patently do not, but Aguero, his 20th goal of the season safely tucked away and Rolando booked for clumsily dragging him back soon after, was unlucky in his search for No.21.
David Silva's hoisted first-time pass after 28 minutes still left him plenty to do. He rounded the goalkeeper yards outside the penalty area then quickly chipped a backpedalling defender only for the ball to come back off the bar.
Five minutes before the break Silva once more sent his team-mate clear, this time with Toure tearing upfield on the left to support him, but when Aguero received the ball back from Yaya, he contrived to fire wide of an inviting target.
Porto would have claimed a second goal was just unfair, for they had shaped up on more than one occasion to find a way through Captain Kompany's rearguard without finding a finish.
Twice Hulk whacked in those low driven crosses that can turn into goals at a touch - see last week's first leg for evidence - and Joe Hart was relieved to watch them zip harmlesssly past his six-yard box.
Joao Moutinho, a tricky opponent, sent a free kick skimming over Hart's crossbar, Hulk sent one sizzling just past a post with the aid of a deflection, and Nicolas Otamendi belted a chance skywards when he should have done better.
Porto did have the ball in the net after 56 minutes but an alert linesman had spotted an offside infringement at the near post as the Portuguese swarmed into Hart's penalty area
While Porto continued to commandeer the ball, City were content to contain them, with Kompany marshalling his defence superbly and the midfield working hard to telling effect.
Then in the final minutes, that astonishing finale as Mancini sent on his substitutes Dzeko and Pizarro and Porto were completely undone in quite ruthless fashion.
Dzeko put the result on the night beyond doubt from Aguero's pass after 75 minutes, in the process inciting a protest from Rolando that saw the Porto player sent off for his second yellow card.
With the visitors a man down and morale slipping, Silva popped up completely unmarked in the six-yard box to tap home the third goal from Pizarro's cross before the on-loan Roma midfielder himself completed the scoring a couple of minutes later.