Active citizens may contribute to the environmental, social, and institutional resilience of cities. This review discusses how citizen initiatives protect biodiversity hotspots, contribute to social cohesion, institutional innovation, and diversity in urban green space management. Challenges related to social inclusiveness, ecological connectivity and continuity suggest government involvement is pertinent, but needs to be refocused. To maximise environmental outcomes of active citizenship, governments may adopt an enabling and stimulating governance style that harnesses the transformative potential of active citizenship. This paper argues for mosaic governance to work with the heterogeneous array of people, institutions, and spatial practices associated with active citizenship. Mosaic governance aims for a context-sensitive way of urban green infrastructure planning, enhancing relationships between the diversity of landscapes and communities across cities.