Mancini was relieved of his duties a year to the day since he guided City to last season's Premier League title.
City's owners spoke of a desire for a more "holistic approach" but Ferguson believes it was the wrong decision.
"It is quite amazing," said the Manchester United manager, who retires after Sunday's game at West Brom.
"He has won the FA Cup, been in the final, second in the league and won the league and it's not good enough?
"I was surprised, but actually no, you can't say you are surprised not with some owners today. You can't be surprised, but I still don't think it was right.
"I was with the chief executive of Liverpool at our reserve game the other night and he told me that Brendan Rodgers, who has not been in the job a year, is the 30th longest-serving manager in the country. That's incredible."
Meanwhile, Ferguson revealed that he is no longer interested in the future of wantaway striker Wayne Rooney and says that his successor, David Moyes, will decide on whether the player has a future at Old Trafford.
He said: "I have not spoken to David about Wayne; I'm sure he will address it when he gets here. I have no idea why Wayne wants to leave, it is not my department. I am no longer interested in that.
"David is going to deal with that and, quite rightly, he wants to deal with it."