The school librarian has a critical role to play in the 21st-century school learning environment. However, because of the constant changes in K–12 school environments and the difficult decisions administrators must make about allocating money to best help students, school librarians must rigorously document their professional growth and impact. Prior research demonstrates that action research is a promising vehicle for the professional growth of K–12 educators. This paper reports on a qualitative investigation on the outcomes of a year-long structured professional development approach that engaged in-service school librarians in action research. Findings demonstrated that action research gave the participants in this study a viable way to pursue professional growth. Results from thematic analyses indicate that the participants experienced improved collaboration with stakeholders, increased support for their school library programming, purposeful reflexivity (that is, intentionally thinking about their own feelings and thoughts in the context of how these could affect their own actions), and personal validation of their roles at the school. We also offer insights to practitioners who may wish to design similar professional development initiatives. Specifically, we have identified potential barriers that school librarians may encounter during the action research process, and have included recommendations for embedded scaffolds and supports for school librarians doing action research and changing their practices based on the results.