<p>Handled by a constellation of public and private institutions, the COVID-19 vaccine infrastructure in the U.S. was largely fragmented when it first emerged. By looking into the early phases of the COVID-19 vaccination distribution among older adults in the U.S. state of Florida, this work attends to the “last-mile logistics” of vaccine infrastructure—the final leg of distribution, where vaccines meet their intended end consumers. Drawing on in-depth interviews with older adults and the volunteers who supported them in securing vaccination appointments, we illustrate the complexities of the last mile and the challenges faced by older adults in navigating distribution. We then demonstrate how volunteers and older adults came together as an emerging assemblage to collectively perform what we term “last-mile work”—a specific form of infrastructuring undertaken in the last mile to resolve localized logistic or operational challenges within a larger infrastructural arrangement. This paper introduces last-mile work as a critical yet under-theorized form of infrastructuring that is indispensable to infrastructure’s everyday function. We argue that last-mile work offers an important analytical lens and empirical site for future infrastructural investigations in CSCW research and design. It creates new entry points to rethink the assumed relationships between infrastructuring work and temporal breakdowns, while also surfacing critical questions of labor, power, and (in)visibility within the last mile of infrastructures.</p>