In Whole Person Librarianship: A Social Work Approach to Patron Services (2019), Zettervall and Nienow aptly refer to librarianship and social work as ‘sister professions’, highlighting their gendered histories and similar professional values. Both librarianship and social work emerged as service-oriented, female-dominated professions in the late nineteenth century. Questions about professional identity have concerned librarians and social workers since those early days, with scholars and practitioners in both fields examining markers of professionalism, such as training, associations, and values. This article compares the historical development of professional social work and librarianship by tracing the changing discourses on professionalism, attending to the role gendered language plays in debates about professional status. The analysis focuses on the works of early pioneers in American social work and librarianship and the public debates about professionalisation in the early 1900s to provide context for collaborations between librarians and social workers and look towards the future.