This paper examines the most effective ways of promoting the sustainability of microfinance institutions (MFIs), as revealed by an in-depth study of five UK MFIs. The research focuses on three key sustainability factors: effective partnerships, staff efficiency and loan portfolio profitability. Through benchmarking and analysing the institutions’ loan portfolios, operational processes and staff time allocation, the study reveals that the majority of UK MFIs are unable to cover their full costs through lending activities alone. The results suggest that sustainability could be significantly improved by setting interest rates that more accurately reflect the costs of service delivery and by enhancing loan officer productivity through greater engagement with potential clients. The study also emphasises the importance of adequate loan capital and development grants in supporting growth and productivity improvements. Overall, the findings provide practical recommendations for funders, policymakers and MFIs, emphasising the value of collaboration and knowledge sharing across the microfinance sector to achieve greater financial sustainability and social impact.