The visitors had taken the lead just before the 15-minute mark via an excellent solo goal from Luis Dias, who picked up play on the left wing before cutting inside and finishing back across Ederson into the bottom-right corner.
And Aguero made no mistake from the resulting penalty, squeezing home his 40th Champions League goal, and first of the new season, under Marchesin.
We started the brighter in the second half, with Porto's goalkeeper once again called into action within the opening four minutes when Gundogan’s placed volley back across goal from a right wing cross looked destined to find the bottom corner.
But the German wasn’t to be denied a second time, and stepped up when Pep Guardiola’s men were looking for a moment of inspiration to overcome a stubborn Porto side.
After being fouled on the edge of the box, the midfielder curled a delightful free-kick up and over the wall from 20 yards out to give us the lead in the 65th minute.
But it was job done for City, who kicked off a tenth consecutive Champions League campaign with a hard-earned three points.
Check out the best of the action via the gallery below…