In the context of a mixed economy of welfare, public policy in the UK and elsewhere has long promoted third sector involvement in delivering public services. A growing research literature consistently highlights the challenges third sector organisations face engaging with a demanding public services commissioning environment, but it tends to lack a theoretical basis and can offer misleading accounts of third sector organisations as relatively passive and powerless in the face of wider forces. This article argues that third sector organisations actively operate within and seek to shape a commissioning context which advantages some strategies and some types of organisation over others. To provide stronger theoretical foundations for understanding public services commissioning and the third sector, the concept of ‘strategic selectivity’ (Hay, 2002) is applied to in-depth qualitative longitudinal data from third sector organisations delivering a range of public services. The article contributes new theoretical insights into the dynamic ways in which social policies and public services are organised. The analysis highlights how differently positioned organisations seek to read, navigate pathways through, and transform an uneven public services commissioning landscape.