The 31-year-old, who works in a tyre warehouse in Bristol, has been assigned the number 12 shirt for the coming season following an innovative campaign by City's sponsors to bring fans closer to the game.
As the winner, Kenna will take part in selective team activities, such as the official team photo and leading the team out for the opening game of the season.
Blackthorn offered all season-ticket holders the chance to become part of the squad for the 2013/14 season.
Speaking at his official unveiling, Kenna said: "I'm thrilled to have won a place in the official City squad. You always expect someone else will be the winner, but I'm over the moon.
"This club means so much to me, especially as my great uncle Alec Eisentrager played for the club between 1949-1958."
Bristol City's commercial director Kevin Smith explained: "We want our fans to get closer to the club that they love more than any other in English football. 'Signing' Craig shows our commitment to our fans and providing them with truly unique experiences."
Kenna will be introduced to fans at the club's first home game of the season, against Bradford on Saturday.