Grassroots initiatives (GIs) are local, yet globally connected networks of activists that generate novel solutions for sustainability. While GIs are often claimed to play an important role for urban sustainability transitions, little research has examined how such innovations consolidate beyond isolated local initiatives. This article summarizes findings from a case study of the collaborative economy grassroots movement in Gothenburg, Sweden, to explore how the movements’ transformative capacity can be strengthened to enable transformations away from the growth-based economy, for the purposes of social-ecological sustainability. Our findings suggest that GIs demonstrate potential for post-growth transformations and that intermediary support and leadership are key to strengthen their transformative capacity. However, we also find that as GIs develop to influence regimes, they may face trade-offs that compromise their autonomy. If considered, we argue that the transformative capacities of GIs can be supported, to enact economic transformations toward a post-growth economy.