Sharing initiatives, things collections or libraries of things are becoming developing features in public libraries worldwide. This article reports on a retrospective case study of “the Sustainable Library,” a now-concluded Danish sharing initiative from 2016 to 2022. “The Sustainable Library” is a unique case as an early example of a public library sharing initiative. Moreover, its explicit focus on sustainability and sharing economy makes it an example of sustainable librarianship. The study offers empirical insights into the life cycle of the sharing initiative and its different phases. Examining the case from its organizational context, the study asks how and why the sharing initiative was developed, what were its preconditions and drivers, and which barriers were experienced along the way. Finally, the study makes room for hindsight reflections on the role and responsibility of libraries in the sustainability agenda.