The 34-year-old utility, who this season became the seventh Cats player to reach 300 games, told his teammates at Friday training that his 14th finals campaign will be the last.
Recruited with the No.28 pick in the 2009 draft, Duncan made his debut the following season and played in their 2011 premiership.
He also was a key member of their 2022 flag-winning team and this year has played nine games for a career total of 305.
His most recent AFL game was the round-18 win over Richmond.
Duncan has been renowned as a skilful player who can fill a number of roles - on the wing, forward or as a running defender.
He finished in the top 10 of the club best-and-fairest count seven times and was a long-time member of the player leadership group.
It is a mark of Duncan's value to the Cats and their amazing consistency that he has played finals in 12 of his 16 seasons.
"Every day I thank Stephen Wells for calling out my name on draft night in 2009," the WA recruit said.
"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."
Former teammate Andrew Mackie, now Geelong's football boss, said Duncan had been an outstanding person and footballer at the Cats.
"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," Mackie said.
"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.
"He will continue to play an important part within our squad over this coming finals campaign."
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