Haaland has scored 21 Premier League goals this season, making him the league's top scorer
The Norwegian striker missed two games through injury but came off the bench in the 62nd minute at the City Ground.
He slotted into the left corner nine minutes later to wrap up the points and move City back to a point behind Premier League front-runners Arsenal, who won at Tottenham earlier on Sunday.
City went ahead 32 minutes in as Josko Gvardiol scored with a powerful near-post header from Kevin de Bruyne's inswinging corner.
That came while Forest's Neco Williams was off the pitch receiving treatment for an injury, which subsequently forced his substitution.
Pep Guardiola's City have a game in hand over Arsenal but may still need to win all of their games between now and the end of the season to secure a record fourth consecutive Premier League title.
This was a far from comfortable afternoon for the visitors as Forest disrupted their flow and created a number of good opportunities.
Based on this performance, it is hard to believe Nuno Espirito Santo's side are in a relegation fight, but they remain just one point above the drop zone.
After a tumultuous time off the pitch in which they have criticised refereeing standards, this was a determined display from Forest but they ultimately paid the price for missed chances.
-- Courtesy of BBC Sport