Phillip Maxwell, 29, of Merseyside, hurled it after City equalised against Aston Villa on their way to a 3-2 win to clinch the Premier League title.
Manchester Magistrates' Court heard a police spotter captured him throwing the missile over the heads of fans and stewards at the Etihad Stadium in May.
Maxwell also received a fine.
He threw the flare when City made it 2-2 in the 78th minute.
Shahid Khan, prosecuting, said: "We would submit there was a clear risk of injury."
He said a total of 20 smoke flares were lit at the City-Villa fixture alone with some hitting supporters, he added.
Chairman of the bench Sohail Ahmed told him: "You have lost your good character and paid a big price for this stupid mistake."
Maxwell, of Anzio Row, Knowsley, admitted the offence and was fined £1,440. He was also ordered to also pay £229 costs as well as getting the football banning order.
In mitigation, Chloe Gaffney said her client accepted the prosecution case but disputed he intended to throw the missile towards the away supporters.
She said: "The defendant himself has described it as an act of immaturity.
"This is an experience he does not want to go through again."
Following sentencing, Kerry Grieve, of the Crown Prosecution Service, said: "Maxwell's actions were thoughtless and selfish.
"Flares create a risk to the safety of players and spectators."