The 33-year-old Smith claimed the individual gong for his eye-catching performance in the Cats' 81-point belting of Sydney in Saturday's decider at the MCG.
The previous oldest medallist was Richmond's Kevin Bartlett, who was 32 when judged best on ground in the Tigers' 1980 premiership.
Geelong's Smith collected 32 disposals and booted three goals, including two in the opening term which helped the Cats set their dominant tone.
Smith also had a whopping 14 score involvements, 11 inside 50s, five clearances, took a dozen marks and recorded a game-high 771 metres gained.
"To our boys, unbelievable," Smith said after collecting his medal.
"I am still in a little bit of shock. It was a phenomenal performance."
The dynamo's feats came in his fourth premiership - he won three as a member of Hawthorn's triple treat from 2013-2015.
The fleet-footed Smith produced his best grand final display for his new club.
But he was also a solid performer in the three Hawthorn flags, collecting 18 disposals in the 2013 grand final, 17 touches in the 2014 decider and 23 possessions featuring three goals in the 2015 finale.
Smith and his Cat captain Joel Selwood are the only current players to win four flags - the VFL/AFL record is Hawk legend Michael Tuck's seven.
And Smith is the fifth Geelong player to win the medal, following Gary Ablett Sr (1989), Steve Johnson (2007), Paul Chapman (2009) and Jimmy Bartel (2011).
Smith moved to the Cats as a free agent after playing 210 games for the Hawks between 2011-20.
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