The National Institute of General Medical Sciences (NIGMS) mandates that its Centers of Biomedical Research Excellence (COBRE) and Institutional Development Award Networks of Biomedical Research Excellence (INBRE) institute formal mentoring programs to promote the core program objective of junior investigator development. Despite this NIGMS requirement, and the many career-related benefits associated with mentoring, few tools exist for purposes of rigorously evaluating COBRE and INBRE mentoring programs. The purpose of this project was to develop a mentoring assessment tool to aid in the evaluation of COBRE and INBRE mentoring programs. In "study 1," a list of items comprising the tool was created via a multiphase item generation process based on input received from subject matter experts within the Cognitive and Neurobiological Approaches to Plasticity Center. In "study 2," feedback about this tool was solicited from 78 grant directors, mentees, and mentors representing 21 unique COBRE programs and 8 unique INBRE programs from across the United States. The results provide initial evidence that this tool possesses suitable psychometric properties, is a flexible instrument with many potential uses, and represents a valuable resource for helping evaluate COBRE and INBRE mentoring programs. Having a tool for evaluating mentoring can help promote the grant success and career development of junior investigators in COBRE and INBRE programs and help program directors develop more sustainable research centers.