PURPOSE: To determine the psychometric properties of the Mentoring Competency Assessment (MCA), a 26-item skills inventory that enables research mentors and mentees to evaluate six competencies of mentors: maintaining effective communication, aligning expectations, assessing understanding, addressing diversity, fostering independence, and promoting professional development. METHOD: In 2010, investigators administered the MCA to 283 mentor-mentee pairs from 16 universities participating in a trial of a mentoring curriculum for clinical and translational research mentors. The authors analyzed baseline MCA data to describe the instrument's psychometric properties. RESULTS: Coefficient alpha scores for the MCA showed reliability (internal consistency). The hypothesized model with its six latent constructs (competencies) resulted in an acceptable fit to the data. For the instrument completed by mentors, chi-square = 663.20; df = 284; P < .001; root mean square error of approximation (RMSEA) = 0.069 (90% CI, 0.062-0.076); comparative fit index (CFI) = 0.85; and Tucker-Lewis index (TLI) = 0.83. For the instrument completed by mentees, chi-square = 840.62; df = 284; P < .001; RMSEA = 0.080 (90% CI, 0.063-0.077); CFI = 0.87; and TLI = 0.85. The correlations among the six competencies were high: 0.49-0.87 for mentors, 0.58-0.92 for mentees. All parameter estimates for the individual items were significant; standardized factor loadings ranged from 0.32 to 0.81 for mentors and 0.56 to 0.86 for mentees. CONCLUSIONS: The findings demonstrate that the MCA has reliability and validity. In addition, this study provides preliminary norms derived from a national sample of mentors and mentees.