Sharing initiatives, things collections or libraries of things have become developing features in public libraries within recent years. This article reports on a retrospective case study of The Sustainable Library, a now-concluded Danish sharing initiative from 2016 to 2022. As an early example of a public library sharing initiative, The Sustainable Library is a unique case. Moreover, its explicit focus on sustainability and sharing economy makes it an early 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 public libraries and sharing initiatives in the sustainability agenda.