St John was appointed to the Fonterra board in 2016 and is currently a member of Fonterra’s audit, finance and risk committee, sustainability and innovation committee, and disclosure committee.
He was previously chair of the milk price panel for over six years.
“I’ve thoroughly enjoyed my time with Fonterra, including working with the board and management team through its strategic reset,” St John said.
“The co-op has seen a real turnaround in its performance and has a bright future.”
St John is the chair of Fisher and Paykel Healthcare and serves on the boards of ANZ Bank New Zealand, Mercury NZ Limited and Next Foundation.
He will step into the role of chair of Mercury NZ Limited from January 1 next year.
Fonterra said it would start a search process to replace St John.
Farmer shareholders will have an opportunity to ratify that appointment at the 2024 annual meeting, it said.