City had hoped to build on last season's Premier League title success by bringing in Robin van Persie from Arsenal, but lost out to rivals Manchester United.
The champions were also linked with other big names such as Javi Martinez, Eden Hazard and Daniele di Rossi but instead brought in Jack Rodwell, Matija Nastasic, Maicon, Scott Sinclair and Javi Garcia to mixed success.
With City now trailing United by 12 points at the top of table with 12 games remaining, their hopes of retaining the title are fading and it could be argued what happened last summer cost them.
Mancini told The Guardian: "It's important to realise we have made some mistakes.
"I have made some mistakes and the players have made some mistakes.
"But the first reason is because we didn't do what we should have done in the summer transfer market - we worked really badly in the market."
Mancini's words could be interpreted as criticism of Brian Marwood, the club's former sporting director who has since switched to a role in charge of the Academy.
Mancini expects better recruitment this summer following the arrival of former Barcelona figureheads Ferran Soriano and Txiki Begiristain as chief executive and director of football respectively.
He said: "Ferran came from Barcelona and understands what it needs to be a top club. Txiki played football and knows football.
"They are good men. For this reason, I am optimistic about our future. We now have people who know their football."
Strikers Edinson Cavani and Radamel Falcao of Napoli and Atletico Madrid respectively could be among City's targets this summer and Mancini admires them both.
He said: "Cavani and Falcao would work in England. They have experience. Both players are 26, 27. They are good enough to play in England."
Sinclair is also yet to make his mark at the club having made just two league starts but Mancini concedes his had not given him enough opportunities.
He said: "I haven't given him a chance. It's not his fault. He's working well every day and he hasn't caused any problems. I'm very sorry."
@peter griffin - that's fine mate. I'm not the only one who sometimes refers to outgoings, and sometimes to net spend, as evidenced by your previous claim that United had spent 4x what City did this season (which uses City's net spend, ignores Ballotelli's fee being payed in instalments, and uses United's gross spend). And You're right about Souness/Evans of course - I had it right below but then had a moment and got them the wrong way around in a later post. But it's a bit shady to now downgrade your claim from 'spending double what their rivals did' to 'spending the same as Liverpool', who started with a better squad. And following our first EPL we didn't outspend the others either, as detailed elsewhere. I don't think we're massively in disagreement - only that we object to each other's headline comments from time to time (not surprising given who we support). Thanks for the bit on Director of Football btw. I hope Fergie takes that role at United at some point in the future, assuming he is able to allow the manager to manage, when he does move upstairs.- big dave