Ellis spent 26 of those years in his current post, but he started as a member of the mutual before becoming an employee.
Over that time, he has overseen an increase in assets from £18m to £226m at the end of 2020.
The mutual said it had its “strongest pipeline of lending ever” resulting from growth in interest in sustainable lending solutions.
The recruitment process for a chief executive replacement has begun and Ellis will remain in his role until 2022 to oversee the handover.
Ellis (pictured) said: “I have had the great good fortune to work for an organisation whose principles and reason for being I passionately support. Indeed, with Ecology I have had the opportunity to demonstrate how balance sheet dynamics can be constructed in order to pursue the provision of sustainable finance.
“Nothing would have been possible without our members and wider stakeholders in the green building community, and I am proud to have met so many passionate people and experienced so many exciting projects. I will be leaving Ecology with fantastic memories to continue the fight for a sustainable future through other means.”
Steve Round, chair of Ecology’s board, added: “I would like to thank Paul for his extraordinary contributions to both the development of the society, as a member, then director and latterly CEO, as well as the wider values-based banking, social and environmental finance, and green building community.
“Throughout that time his unwavering commitment to our founding members’ vision of applying ecological principles to finance to deliver a sustainable future has guided the society from its early pioneering activity to our current position as a leading exponent of green finance.”
