Barton had requested a personal hearing and an independent regulatory commission found the 29-year-old guilty of two counts of violent conduct.
The controversial midfielder was shown a red card at the Etihad Stadium for elbowing Carlos Tevez, and he then kicked Sergio Aguero before attempting to headbutt Vincent Kompany.
The FA announced Barton received an automatic four-game suspension, with the additional eight matches a consequence of the manner of his dismissal. He was also fined £75,000.
QPR will now begin their own internal inquiry into what transpired on a dramatic final day of an extraordinary Barclays Premier League campaign.
Club spokesman Ian Taylor said on Twitter: "QPR will be making no comment in regard to the Joey Barton case. Internal investigation will begin in due course."
Barton denied the charge of violent conduct against Kompany, but the FA said the case against the player was found to be proved.
The acts were condemned by the chairman of the regulatory commission, who stated: "There are rules of conduct that should be adhered to, and such behaviour tarnishes the image of football in this country, particularly as this match was the pinnacle of the domestic season and watched by millions around the globe."
The FA's regulatory panel had discretionary powers to further increase the length of the suspension if charges were found to be proven.
Barton's admission on Twitter that his violent actions were a cynical ploy to provoke a City player to be sent off saw those discretionary powers employed.
Watch: Barton handed 12-game ban... More Videos
Barton insisted via Twitter on the day of his sending-off that he had not lost his head, but that a team-mate "suggested I should try to take 1 of theirs with me".
QPR are set for a pre-season tour of Asia in July, but it remains to be seen whether Barton, signed on a free transfer from Newcastle on a four-year deal in August 2011, will be part of Mark Hughes' squad.
Speaking earlier this month, QPR chairman Tony Fernandes said: "There are experienced people who will come back to me and we'll review the whole situation.
"Joey has been an integral part of the club and has played his part.
"This is football, and I'm new to it. I don't profess to have all the answers."