Two research streams examine how social movements operate both “in and around” organizations. We probe the empirical spaces between these streams, asking how activism situated in multi-organizational contexts contributes to transformative social change. By exploring activities in the mid-1990s related to advocacy for domestic partner benefits at 24 organizations in Minneapolis–St. Paul, Minnesota, we develop the concept of inhabited ecosystems to explore the relational processes by which employee activists advance change. These activists faced a variety of structural opportunities and restraints, and we identify five mechanisms that sustained their efforts during protracted contestation: learning even from thwarted activism, borrowing from one another’s more or less radical approaches, helping one another avoid the traps of stagnation, fostering solidarity and ecosystem capabilities, and collaboratively expanding the social movement domain. We thus reveal how activism situated in multi-organizational contexts animates an inhabited ecosystem of challengers that propels change efforts “between and through” organizations. These efforts, even when exploratory or incomplete, generate an ecosystem’s capacity to sustain, resource, and even reshape the larger transformative social change effort.