The shock deal came to light on Tuesday, when Hargreaves underwent an extensive medical, which he came through with flying colours.
Now he has committed himself to the Champions League challengers for the current campaign, knowing he must prove doubters wrong about his awful fitness record.
The 30-year-old has barely played a game since being forced to undergo operations on both knees to cure a debilitating tendinitis problem.
Prior to that, Hargreaves had been an England regular, winning 42 caps. He had also won the Champions League with Manchester United in 2008, having already become only the second English player to win the prestigious trophy at a foreign club, with Bayern Munich.
Such is Hargreaves' talent, there was even talk he might make Fabio Capello's World Cup squad last summer if he could prove his fitness.
However, after lasting just five minutes of his only Premier League game, against Wolves at Old Trafford in November when he tore his hamstring, the Calgary-born player then dislocated his shoulder and failed to recover.
Sir Alex Ferguson decided it was no longer worth investing any time in Hargreaves and opted against offering him a contract extension.
Others have not been put off though.
West Brom were the first to make their interest known, whilst Aston Villa and Tottenham were also considering whether to make a move before City pounced.
Although there had been no time pressure on the deal due to Hargreaves' status as an out-of-contract player, by tying it up quickly, the Blues have ensured their newest recruit will be eligible for European combat, which includes a couple of poignant meetings with Bayern, the first at the Allianz Arena on September 27.
Hargreaves' arrival will complete Mancini's extensive summer recruitment drive, becoming the Italian's sixth signing, all of them internationals, ensuring the Blues will not be lacking in experience on the tough road ahead.