Engaging, accessible, evidence-based interventions are needed to support the professional development of research mentors within the clinical and translational sciences. This article reports on the usage and impact of the University of Minnesota Clinical and Translational Science Institute’s online mentor training module, Optimizing the Practice of Mentoring (OPM). Among the 1,124 OPM users in our contemporary evaluation sample (Feb 2019–June 2022), retrospective pre-to-post gains were observed in respondents’ self-rated mentorship skills (11 items), perceptions of the overall quality of mentoring they provide, and mentoring confidence. A high proportion (83%) of users reported making or intending to make changes in their mentoring practices as a result of the training. Example behavior changes included a greater focus on aligning expectations, more proactive attention to the relationship (overall and its distinct phases), increased usage of active communication skills, adoption of tools such as Individual Development Plans, and ongoing self-reflection. Over a 10-year period, 4,011 unique individuals registered for the module, representing over 650 different institutions (a majority being affiliated with past or current Clinical and Translational Science Hubs). OPM has the versatility to be employed as a standalone, asynchronous approach for mentor development or as one curricular component of more comprehensive, multimodal programs.