Jim McCarthy will be succeeded at the annual general meeting on March 23 by Steve Ellwood.
Mr McCarthy is a former chief executive of Willenhall-based Poundland and has been on the board of the group, which has its headquarters at Llansantffraid, Powys, and stores across Shropshire, since 2011.
In order to ensure a smooth transition, Mr McCarthy will remain on the board as a non-executive director until the end of July, when he will retire.
Mr Ellwood, who has been a non-executive director since 2016, has significant experience of the UK agricultural sector, having previously been head of agriculture at HSBC for 10 years, and head of food and agriculture at Smith & Williamson.
He also holds non-executive directorship positions at: NIAB Group, the crop science and research organization; AH Worth, the fresh produce group, where he is chairman; and Velcourt, the international farm management group.
It is the Wynnstay board's intention to appoint an additional non-executive director, and it has started a recruitment process to identify suitable candidates.
"After 10 years on the Wynnstay Board, I step down, confident about the group's growth prospects. The business has a strong management team, clear growth strategy, and robust balance sheet.
"I am delighted to handover to Steve. He is highly experienced, with significant knowledge of the sector as well as of Wynnstay. I have no doubt that he will help to drive the successful execution of the board's strategy, and wish him and all my colleagues every success,"! said Mr McCarthy.
Mr Ellwood said: ""On behalf of the board, I would like to thank Jim for his significant contribution to the board and to Wynnstay over the past 10 years. His commitment to the group has been outstanding.
"I am excited about the Group's prospects. Wynnstay is very well-positioned, both operationally and financially, as the agricultural sector embarks on a transition set by Brexit and the UK Agriculture Bill. We believe that there is a clear opportunity for Wynnstay to assist UK farmers to feed the nation in a more sustainable, environmentally-friendly and profitable way."