This article details a two-phase study of the development and validation of a mentor attunement scale (MAS). Phase 1 involved theory-driven generation of 20 items to measure four facets of attunement. Rasch analysis of data collected from 390 youth mentors indicated problematic reverse-coded items, which were eliminated to improve scale functioning. Exploratory Factor Analysis and reliability analysis applied to the same dataset supported this removal, and pointed to an internally consistent, 12-item, one-factor solution, then tested using Confirmatory Factor Analysis (CFA) in the second phase. CFA applied to a second cohort of 453 mentors produced a good fitting, single-factor, 5-item scale. A measurement model consisting of the five attunement items and an existing, 7-item empathy scale was then tested to confirm convergent and discriminant validity. The final solution supported a two-factor, 9-item MAS. Use of the MAS can advance the fragmented conceptualization and measurement of critical mentoring skills.