The Norwegian forward broke the deadlock shortly after the restart before sealing the points in stoppage time, allowing Enzo Maresca to begin his Champions League reign as Manchester City manager with victory.
The result was particularly significant given the atmosphere and Porto's resistance at the Dragão, where City had to withstand periods of pressure before finding the decisive goals.
Porto Frustrate City in First Half
Manchester City controlled much of the opening period but were unable to turn their possession and chances into a goal.
Diogo Costa was outstanding for Porto, making a series of important saves to keep the Portuguese champions level.
Haaland came close in the 20th minute when he met Phil Foden's cross with a powerful header, but Costa reacted sharply to push the effort behind.
The City striker had another excellent opportunity shortly afterwards when Matheus Nunes' low cross found him in front of goal. Costa again produced the necessary intervention.
Foden and Antoine Semenyo also threatened as City searched for an opening, but Porto survived the pressure and the teams went into half-time 0-0.
Haaland Breaks the Deadlock
Manchester City needed barely a minute of the second half to finally find the breakthrough.
Haaland opened the scoring in the 46th minute, rising to meet a cross from Enzo Fernández and directing his header beyond Costa and into the net.
The move began with Rayan Cherki, who produced a clever touch before Fernández delivered the decisive ball for Haaland to attack.
Porto responded immediately and began putting City under considerable pressure.
The hosts came close through several attacks, while City goalkeeper Gianluigi Donnarumma was forced to remain alert as Porto searched for an equaliser.
Porto Push, City Hold Firm
The home side enjoyed a sustained spell of pressure after falling behind.
Gabi Veiga and other Porto attackers tested the City defence, while the hosts also appealed unsuccessfully for a penalty after an incident involving Donnarumma and Pepe.
City were forced to defend deeper than they would have preferred, with Maresca making changes to restore control.
Elliot Anderson and Iliman Ndiaye were introduced in the second half, while Rayan Aït-Nouri and Mateo Kovačić also came on as City looked to manage the game.
Despite Porto's pressure, City's defence held firm.
Haaland Seals the Victory
Porto's hopes of rescuing a point finally disappeared in the 91st minute.
After a Porto corner was cleared, City retained possession and Aït-Nouri burst forward before cutting the ball back into the area.
Haaland calmly finished into the far corner to make it 2-0 and complete his brace.
The goal gave City the breathing room they needed and effectively ended Porto's hopes of a comeback.
Haaland was later involved in another heated moment with Porto midfielder Pablo Rosario, with both players receiving yellow cards after a confrontation in stoppage time.
Maresca Makes Winning European Debut
The victory gave Enzo Maresca his first Champions League win as Manchester City manager and extended his perfect start since taking charge, with City having won their opening four matches under the Italian according to the latest reports.
Maresca also handed Ayyoub Bouaddi his Manchester City debut, with the young midfielder starting alongside Fernández.
The 17-year-old impressed in the centre of midfield, with Maresca later describing his performance as exceptional and praising the composure he showed in a demanding European environment.
City Begin Champions League Campaign with Three Points
The 2-0 victory gives Manchester City three points from their opening league-phase fixture and an important early boost in the new Champions League campaign.
Porto, meanwhile, were left frustrated after competing strongly for long periods but failing to capitalise on their second-half opportunities.
For City, the biggest positives were the clean sheet, the resilience shown under pressure and, once again, the decisive finishing of Haaland, whose two goals settled a difficult European away fixture.
Manchester City's next Champions League assignment comes against Paris Saint-Germain on 14 October, while Porto travel to Spain to face Real Betis on the same date.