No official announcement has been made by the Eastlands outfit, but a video clip, which has since been removed from the site, shows the 23-year-old defender having a medical.
The France international said: "I am here to help the team win titles, the league, the cup and the Champions League. I am ambitious and this is why I am here."
He added: "(My ambition is) to play as many games as possible and to help the team the best I can to be champions of England."
City confirmed that the deal had not been completed, while it is understood the leak could be due to hacking.
Mangala joined Porto in 2011 from Belgium club Standard Liege and has made 92 appearances in total.
He made his international debut last month in France's 1-0 friendly defeat to Uruguay and was part of Didier Deschamps' squad for the World Cup in Brazil, although he did not feature.
- Sports & Recreation