He has been the mutual’s chief executive for 26 years, which makes him the longest serving CEO in the sector.
In a statement, the Silsden-based building society said: "Over that time, he has overseen an increase in assets from £18m to more than £226m at the end of 2020.
"With its strongest pipeline of lending ever, the society is experiencing a strong interest in its sustainable lending solutions and is well-placed to support efforts to deliver a green recovery as the Government prioritises efforts to tackle the climate and ecological emergency in the run-up to COP26.
"Ecology’s environmental lending has never been more relevant as a progressive force for positive change.
A spokesman said: “The society has started the recruitment process for a new CEO and Paul is expected to remain in post until 2022.”
Mr Ellis added: "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.”