The Norwegian has now scored 12 goals in seven competitive games for City since joining the club from Borussia Dortmund during the close-season.
The goals also made the 22-year-old the fastest player to score 25 goals in the Champions League - a milestone he has reached in just 20 matches.
The blond striker put City ahead in the 20th minute, stretching to reach a cross from Kevin de Bruyne and place it past Sevilla keeper Bono.
Phil Foden made it 2-0 after the break, jinking and turning to create space on the edge of the box before placing the ball into the bottom corner.
Ilkay Gundogan had an effort disallowed after turning in a header from Foden, who was in an offside position but Pep Guardiola's side soon had the margin of victory their dominance deserved.
Haaland showed his goalscoring instinct to make sure of a comfortable victory, pouncing to tap-in after Bono had pushed out a low drive from the impressive Foden.
Even with victory assured, City continued to push forward and defender Ruben Dias added the fourth in stoppage time, turning in a low cross from Joao Cancelo.