The decision comes fresh after City's disappointing FA Cup final performance, losing 1-0 to Roberto Martinez's Wigan Athletic to complete a trophy-less season.
Here are five errors the Italian made which contributed to his dismissal.
Things started badly for Manchester City when a frustrating summer transfer window bore little in terms of fruit.
Robin van Persie and Daniele De Rossi were the targets, but the Citizens ended up signing a disappointing haul in Javi Garcia, Jack Rodwell, Richard Wright, Scott Sinclair and Maicon.
The lone bright spot was Matija Nastasic, who has been fantastic, but the others were bad at face value and arguably worse on the pitch.
Sinclair made two Premier League starts.
Roberto Mancini had the right idea in freshening up his approach ahead of the new season: Manchester City were coming in as defending champions, and thus faced an extra level of scrutiny.
Everyone wants to beat the champions, everyone wants to claim a scalp.
The Italian switched it up, training with a 3-5-2 formation in preseason and carrying it into the Community Shield match.
He abandoned it, sticking squarely to the 4-2-3-1 formation, but with no true wingers it was too narrow, inviting problems.
Scott Sinclair was perfect here but never used, and James Milner never given a consistent role. Problems, such as the bottle-necking against Wigan at Wembley, frequently occurred.
According to the BBC, poor relationships with those inside the organisation was a major factor in Roberto Mancini's dismissal.
He has developed a reputation for blaming anyone he can other than himself when things go wrong, and whether or not that's true, it's tough to defend his mini-attack on the board after failing to sign Robin van Persie (via The Metro).
Manchester City appointed Txiki Begiristain as director of football midway through this season, and Mancini appears to have made the fatal mistake of not cozying up to him.
The BBC's David Bond has also reported that Roberto Mancini's failure to pay attention to the youth-development movements at the Etihad Stadium is another reason for his sacking.
This argument is tough to sustain, though, due to the size of the Manchester City squad.
Coming into the season as champions, the pressure was on; Mancini had no choice but to go full-strength (as the laws of the game dictate) in every single game to avoid a shock loss and subsequent media outrage.
This left him very little room to give youth products minutes on the pitch, and he had a very big squad—encompassing two senior players for every position bar the wings—to keep happy.
Mancini tried to give his youth products a chance, fielding Denis Suarez, Abdul Razak and Jeremy Helan in the League Cup against Aston Villa. The Citizens lost 4-2.
The last straw, the one that broke the camel's back.
Most Manchester City fans would have accepted an FA Cup win, UEFA Champions League qualification and a second-placed finish as a good season. Shelve it, improve, enjoy the cup win, move on.
But losing the cup final to Wigan Athletic was not in the script, and in a game in which many fancied City to deliver a thrashing, Roberto Mancini was out-coached by Roberto Martinez.
For the board, who clearly weren't keen on the Italian after a breakdown in relationships, a trophy-less season represented the perfect excuse to remove him from his position.
City fans were vocal and clear at Wembley in their support for the boss, and only a world-class appointment will appease their sense of betrayal.