The barriers to concerted political action on climate change mitigation are steep, especially in multilevel systems where power is diffused and authority contested. This article seeks to explain how mobilization—galvanizing resources and people to participate actively—occurs in complex multilevel systems. It compares two different polities—the United States and the European Union—to tease out the key features of multilevel systems and how they affect climate activism and mobilization. To capture this dynamic, it proposes a three-staged model of mobilization: awareness building, alliance building, and network creation. The latter stage features “mobilization networks”—stakeholder networks able to transcend levels and institutional inertia and steer polities toward particular climate goals. The article demonstrates how each stage of mobilization is highly contingent on stakeholders' ability to exploit—or at least navigate—multilevel institutional barriers.