Garden Organic chief executive Myles Bremner is leaving in August to become a food and community sector consultant after six years at the charity.
Garden Organic employs 60 staff and has a 30,000 membership.
Bremner said: "I have greatly enjoyed my time at Garden Organic, working with inspiring members, staff and volunteers in taking our work out to new audiences.
"It is especially encouraging to see the progress we have made in demonstrating and proving that gardening and growing has real, profound benefits for people’s health and wellbeing, the community and for the environment
"I look forward to being able to work with and support Garden Organic’s work in the future, as I continue to work in the food and community sector."
Garden Organic president Tim Lang, professor of food policy at City University, said: "Myles has led the organisation through some challenging times, but has helped to place Garden Organic as an authoritative organisation supporting and leading on great innovation and ideas.
"Garden Organic has contributed important policy advice under Myles’ leadership - in his role Chair of the Food Growing in Schools Taskforce, and through the recent school curriculum review, where horticulture is mentioned for the first time. He has helped to make practical gardening and growing accessible to many new audiences."
Garden Organic trustees are now recruiting a new chief executive.