Whitmore joined Midlands as a shelf stacker back in 1964, when it was the Birmingham Co-operative Society, and rose up the ranks to become head of food operations in 2005.
"I'm proud to have been part of a business that genuinely puts people first, whether it be our fellow colleagues or the customers we serve in our stores," he said.
"I'm delighted to be leaving a successful and growing business as we roll out the now nationally recognised The Co-operative branding across our estate. Our stores are looking fantastic with the new signage, fixtures and fittings and I'm really confident the society will continue to go from strength to strength."
Whitmore intends to enjoy more time with his family as well as indulge a love of travelling.
"Pete's story is an inspiration to others in the society who wish to develop careers with us," said Midlands Co-op general manager of retail Tony Carroll. "These are exciting times as we continue to grow and achieve excellent results, despite the current economic climate, and Pete's decision-making has had its part to play in this."
Two weeks ago Midlands Co-op reported a 6% increase in trading profits to £16m on sales up 10% to £503m for the six months to 8 August.