The Italian conceded he had been complacent having watched his side's three-point lead at the top of the Barclays Premier League get wiped out following a 1-0 defeat at Goodison Park.
Former Manchester United midfielder Darron Gibson, whose old club now trail the leaders only on goal difference, scored the only goal of the game on the hour.
Mancini said: "I didn't prepare well for this game. I thought it was maybe easier. I know here it is never easy, never.
"I acknowledge my mistake. It is important for me to know this because, like this, I can't do the same mistakes in the next game."
When asked to elaborate on his mistakes in his post-match press conference, Mancini referred to the team's training in recent days.
He said: "The players put everything on the pitch but I made some mistakes during the last three days preparing for this game.
"I didn't prepare very well. For this reason we lost this game but I also feel we didn't deserve it because they had two chances to score and we were so unlucky in the same situation."
City have won just one of their last 14 visits to Goodison Park but despite that poor record, Mancini claimed he had been lulled into a false sense of security.
Mancini said: "We played well against Tottenham, we won the last three games in the Premier League.
"I thought Everton were without three or four important players, and maybe we did these mistakes. It can happen."
Mancini claimed he was not concerned about the situation at the top of the table with United closing in.
He said: "This is not important. We are on the top again. We have 15 games. It is important next Saturday we win the game at home. We have time."
Everton boss David Moyes felt his side's victory was one of the best he had been involved with in his near 10 years at the club.
The Scot, already buoyed by the capture of striker Nikica Jelavic, said: "That is as good as it has been at Everton for a long time.
"With the team we had out, the players we had missing, it says a lot about the club and how we always seem to come back to do it.
"We might have played smoother football and been better, but against the side we were playing against tonight, I think the players deserve a lot of credit.
"Man City are a terrific team. We had to defend for long periods, especially in the second half.
"We limited them to not too many chances, but they have always got a threat with the likes of (David) Silva or (Samir) Nasri.
"So, it was a great effort by the players, who showed a great desire to get a result."