•Stockholm’s Hammarby Sjöstad and Stockholm Royal Seaport were studied.•A mixed-methods approach measured the impacts of eco-districts for cleantech firms.•Eco-district development in Stockholm has supported regional cleantech growth.•Collaborative initiatives led by the City in each project promote innovation.•Monitoring the firm-level impact of urban development projects is recommended. In cities around the world, achieving greater environmental sustainability is increasingly accompanied by economic motivations. This is illustrated in Stockholm, Sweden, Europe’s first Green Capital (2010), where an important aspect of the city’s contemporary environmental success has been the development and promotion of flagship eco-district projects. Packaging the environmental sustainability and marketability of these development projects has been part of a concerted strategy in Stockholm, one shared by numerous cities around the world. But to what extent can eco-districts contribute to economic growth? Despite implicit policy motivation, a concise approach to connecting and measuring these issues has yet to be established. Based on an examination of two eco-district developments, this paper combines a qualitative commentary consisting of company-specific interviews with and analysis of productivity of regional cleantech firms in an effort to verify the connection between Stockholm’s eco-districts and the growth of its cleantech sector. Through this mixed methods approach, promising opportunities to assess the economic impact that eco-districts can have on local eco-innovation sectors have emerged. This type of information can be used to support wider public initiatives in eco-district projects.