On paper, this appeared a tricky match for Pep Guardiola's side, though it was made slightly easier without a raucous crowd to cheer on the hosts.
However, City raced out of the tracks and were in control for the majority of contest with the opening goal coming on 18 minutes as Ferran Torres netted for the second consecutive game in the competition.
The England man made it 3-0 with a tap-in from close range nine minutes from time after an inch-perfect pass from De Bruyne.
Guardiola was pleased with his team's display and told BT Sport: "We played really well, we didn't concede much and we were in control, patient and aggressive without the ball. I'm so satisfied with the performance and result, it's always difficult to win away.
"We can play on both sides, we tried to play high and wide to stretch the pitch. Raheem Sterling was exceptional, Phil [Foden] as well. We didn't expect them to play five at the back until we saw the team. Porto was the same. As much as we play against five at the back we started to take the control and rhythm.
"We were so stable defensively. Aymeric [Laporte] was good, Kyle [Walker] is in the best form since we've been together, Oleksandr [Zinchenko] helped us, John [Stones] played good minutes. I'm so satisfied with the way we played."