This study explores the key determinants of intermediary performance in promoting corporate sustainability in small and medium‐sized enterprises in Africa. Despite being a critical link in the delivery of sustainability support services to small and medium‐sized enterprises, intermediaries have only received limited research attention. Research exploring intermediaries of corporate sustainability in small and medium‐sized enterprises have focused broadly on their roles but narrowly on performance. Those exploring intermediary performance have often focused on larger companies, sometimes, neglecting the specifics of small and medium‐sized enterprises, especially in African countries. Improving intermediary performance in promoting corporate sustainability requires an in‐depth understanding of the factors that determine their performance. As a qualitative study, interview responses from 32 experts working in National Cleaner Production Centres in four African countries were analysed using inductive thematic approach. The findings suggest that programme constraints, external profile, small and medium‐sized enterprises context, impact strategy, and service networks are the key determinants of intermediary performance in promoting corporate sustainability in small and medium‐sized enterprises. These determinants facilitate and inhibit intermediaries’ ability to promote corporate sustainability in small and medium‐sized enterprises. The implications concern the need for intermediaries and small and medium‐sized enterprises to manage these determinants in promoting corporate sustainability.