This study introduces the complementarity perspective to the entrepreneurial ecosystem (EE) research as a process to compare the two entrepreneurial ecosystems of Reading (United Kingdom) and Istanbul (Turkey). We apply the multiple-case design methodology based on the rapid ethnography and the grounded theory to construct two entrepreneurial ecosystem models characterized by different EE taxonomy pillars and embedded in different local and institutional contexts. As a result of the study, two findings emerge. First, we find three distinct complementarities in both EEs: access to resources, effective use of resources, entrepreneurial orientation, and ecosystem awareness. Second, EE taxonomy pillars serve as mediators between ecosystem elements that drive entrepreneurial activity and complementarities. We argue that this approach could be applied to understand how EEs work and develop in regions with heterogeneous economic, geographical, and institutional contexts.