Mancini's men were unfortunate to enter the break a goal down but turned the game on its head thanks to an own goal from Alvaro Pereira and a late strike from the Argentinian
The visitors saw much of the ball in the first half but were unable to create any clear-cut chances and were punished on the stroke of halftime by Silvestre Varela from close range.
City enjoyed a much better second half and was gifted an away goal thanks to the unfortunate Alvara Pereira’s own goal. Porto pressed from then on but was unable to break down the impressive City rearguard and was ultimately punished on the break by Sergio Ageruo.
The home side made just two changes to the squad which cruised to a 4-0 win over U. D. Leiria over the weekend. Eliaquim Mangala and Marc Janko made way for James Rodriguez and Rolando.
The visitors made five changes to the team which took all three points from Villa Park on Sunday. Yaya Toure was drafted straight into the team following his arrival home from The African Cup of Nations, James Milner dropped to the bench.
Meanwhile, Gael Clichy and Micah Richards replaced Pablo Zabaleta and Aleksandar Kolarov in the fullback positions, Samir Nasri replaced Adam Johnson, and Mario Balotelli was chosen to lead the City line ahead of Sergio Aguero.
The Portuguese giants started the better of the two sides, but the City defense did well to stifle any openings, with the only chance of note coming via a Rolando header which Clichy did well to head over the crossbar.
The close shave sparked City in to life as it began to settle and had a flurry of opportunities to grab a crucial away goal.
First, Balotelli was thwarted by Helton in the Porto goal after he flicked on a Nasri free kick. Richards fired a half-volley at goal which was smartly saved at the front post before Nasri was denied by the Brazilian shot-stopper.
City was left to rue those missed opportunities as the home side took the lead just minutes later.
Following good work down the left hand side from Hulk, the Brazilian cut an inviting ball across goal, where Valera did well to steal a march on Clichy before slotting past the helpless Joe Hart.
Mancini’s men responded immediately and really should have restored parity.
Nasri clipped an intelligently lofted ball through to Balotelli and, with the Porto defense non-existent, the Italian had all the time in the world to measure a shot at goal but was again denied by the fantastic Helton.
Both sides exchanged blows with Hulk and Silva both coming close, but it was the home side that entered the break ahead.
City emerged for the second half with renewed vigor and was a width of the post away from an equalizer.
Silva slipped a beautifully weighted ball through to the marauding Richards, who sent his acute effort crashing against the Porto woodwork.
However, Mancini’s men didn’t have to wait long for their second half dominance to pay.
Toure launched a hopeful ball forward towards the isolated Balotelli and with Helton rushing from his line, defender Pereira decided to take charge of the situation. Under pressure from Balotelli, the Uruguayan inadvertently nudged the ball with his shoulder past the onrushing ‘keeper.
Balotelli was reluctant to celebrate his part in the goal as the inquest began between defender and goalkeeper.
The goal sparked the home side in to life as they sensed the need to take a lead to Eastlands next week.
However, the City backline was holding strong, reducing the home side to speculative long-range efforts from Hulk and Rodriguez.
With the first leg nearing its conclusion, City struck a massive blow to Porto’s Europa League chances.
Toure picked up the ball and drove at the Porto defense. With Pereira backing off, the Ivorian advanced into the box before squaring for the unmarked Aguero to hand his side a crucial away win ahead of the second leg next Wednesday.