Tohill played just one game for the Magpies in his three seasons at the club and has decided to turn down a one-year extension tabled by Collingwood for the 2022 season.
The 21-year-old is reportedly set to pursue a career in medicine as he returns home to Ireland, where he will study at Queens University.
“Since Anton arrived at the club he has displayed great enthusiasm, patience and dedication to his game, which in turn was rewarded with an AFL appearance this year,” Magpies football boss Graham Wright said, per the club's announcement.
“Although his stay has been shorter than planned, we appreciate his intentions to resume his studies and return home.
“I’d like to thank Anton for his input to the Collingwood Football Club and wish him the best with his future endeavours.”