South Sydney full-back will remain at the Rabbitohs until at least the end of 2021 after the club activated a clause for the second season in his two-year contract.
The 22-year-old made the move to the Rabbitohs from rivals Sydney Roosters in the off-season and started the first two rounds of the year at full-back.
Mitchell, who made his first-grade debut in 2016, is just two away from 100 NRL appearances having already scored 65 tries.
The Indigenous All Stars representative has earned four caps for Australia and played four games for New South Wales in the Origin arena.
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