The 23-year-old, who posed for photographs and signed autographs outside the Etihad Stadium on Wednesday, is set to sign a reported five-year contract worth ?200,000 a week to finalise his move from Atletico Madrid.
After a day of talks at the Etihad Stadium, Aguero retreated to a city centre hotel and then announced on Twitter that the deal was done.
"I'm already a City player. Happy to be in this club and this city. Thank you all for the welcome and the reception!" he wrote, although minor details of the deal were still to be completed.
His transfer is believed to be costing City in the region of ?35million, which was the amount of the buy-out clause written into the contract he only signed with the Spanish side in January.
On Thursday, Aguero, the son-in-law of Argentina icon Diego Maradona, appeared to take everything in his stride after flying in from Buenos Aires via Heathrow.
After touching down in the north-west, Aguero wrote on Twitter: "Just arrived in Manchester to finalise the details of signing with City.
"Everything is going very well! I'll tell you more in a while."
Aguero had a medical at a private Manchester hospital and briefly stopped off at the Etihad Stadium before going out again for lunch and returning in the early evening for further discussion.
City's previous record buy was Robinho, who arrived from Real Madrid for ?32.5million as new owner Sheikh Mansour, a member of Abu Dhabi's ruling family, made a marquee signing to signal his intent just days after taking over in August 2008.
The cost of signing Aguero will not be a stumbling block for City, who have spent millions revamping their squad and the area around the stadium with much grander plans to come.
With City striker Carlos Tevez adamant he wants to leave to be nearer his wife and two daughters in Argentina, Aguero is seen as the ideal replacement.
Younger than his compatriot by four years, the Atletico star has been a prolific goalscorer in Spain's Primera Division and that is why he became one of the most sought-after strikers in Europe.
Atletico had resigned themselves to the fact they would lose their prized asset when he announced in May, before departing to play for Argentina in the Copa America, that he wanted to leave.
Aguero's preference was to remain in Spain, with city rivals Real seemingly the most likely destination, but Atletico were keen to resist such a move.
The offer from City represented a better option for the Madrid side, although Italian club Juventus were also interested.
Source: Team Talk
Source: Team Talk