Duncan played in three AFL Grand Finals for the Cats, winning the flag in 2011 during his second season of AFL football and in 2022 as well as being a part of the 2020 side that lost to Richmond at The Gabba.
The Western Australian made his WAFL debut for East Perth at age 17 before being selected in the 2009 national draft by Geelong with pick 28.
He would then represent the Cats on a total of 305 occasions, attaining AFL life membership earlier this year.
“After 16 years at AFL level, I feel now is the right time to hang up the boots,” Duncan said in a club statement.
“Every day I thank Stephen Wells for calling out my name on draft night in 2009. I’m grateful to have been part of this Club, a Club filled with so many great people past and present, who have not only taught me about football, but have also helped mould me into the person I am.
“At the end of our season I will sit back and reflect more, however I know that I couldn't have enjoyed the career I have if it wasn't for my wife, children and immediate family for always supporting and giving me the best opportunity to have success in my career.
“While I’m making this decision now, over the next month I’m focused on doing everything I can to help the Cats win its next premiership, whether that’s on or off the field.”
Former teammate Andrew Mackie, who now serves Geelong as their general manager of football, hailed Duncan's immense contribution.
“Mitch has been an outstanding person and footballer to have at our Club over the past 16 years,” Mackie said.
“A trademark over Mitch’s 305 games to date has been his skill and high football IQ, allowing him to play with such flexibility for whatever the team has required, and continues to be a true team-first player.
“Mitch’s value has not been just with how he plays the game, but with how he has shared his knowledge and experience with the younger generations of Cats players who have come through the ranks over his career and he will continue to play an important part within our squad over this coming Finals campaign.
“While there will be a time to celebrate Mitch’s career at Geelong at the end of our season, we thank Mitch, Demi, Scarlett, Ollie, Archie and Parker for their contribution to the Club.”