The Manuel Pellegrini tutored side overcame their Italian challengers 5-4 on penalties after regulation time ended 2-2.
With the presence of superstars Yaya Toure, David Silva and Raheem Sterling, the Nigerian still managed to standout in the match as the Flying Eagles star scored once and assisted the other.
The teenager played as a centre forward, but had to drop deep to make his first big contribution.
With vision well beyond his tender age, the 18-year-old picked the ball in the centre of the pitch barely three minutes into the match, spotted Sterling’s run and lifted a glorious pass to send the new signing clean through.
The former Liverpool winger gratefully accepted the pass and slotted past the Roma keeper, Morgan De Sanctis, for his first Man City goal.
Roma were level five minutes later through Miralem Pjanic, but Iheanacho gave City the lead again.
The vibrant Nigerian was one of only few City players who returned for the second half having started, and he justified Pellegrini’s decision.
Iheanacho had seen a close range header saved by DeSanctis in the 33rd minute, but he gave the Roma keeper no chance when he was through on goal following a back pass six minutes after the break.
Left with only the keeper at his mercy, Iheanacho kept his head, picked his spot and blasted the ball into the roof of the net for City’s second goal.
He was subbed off to a standing ovation from appreciative City fans after 73 minutes.
Adem Ljajic’s 87th minute free kick restored parity for Roma and ensured that regulation time ended in a thrilling draw.