Balotelli, 21, accepted the punishment - believed to be around £120,000 - after being pictured outside a strip club in Liverpool in the early hours of Friday morning, contravening club regulations ahead of Saturday's game against Bolton.
Balotelli went on to play and score in the Premier League leaders' 2-0 defeat of the Trotters at Eastlands but Mancini expressed his disappointment and pledged to investigate.
Mancini said: "I spoke with Mario and gave him a fine of one week's wages for what he did. This is normal because every player should have good behaviour before the game.''
When asked if Balotelli had accepted the fine, Mancini said: "Yes.''
It is the second time Balotelli has broken a curfew this season - among a host of other high-profile misdemeanours - but he admitted in an interview published on Tuesday that he had made a mistake.