Researchers suggest that certain supportive behaviors of mentors could increase the benefits of school-based mentoring for youth. However, the literature contains few validated instruments to measure these behaviors. In our present study, we aimed to construct and validate a tool to measure the supportive behaviors of mentors participating in school-based mentoring programs. The mentor behavior scale (MBS) was developed drawing on the premises of the mentoring sociomotivational model. Two hundred and fifty-three (253) college students participating in an eight-month school-based mentoring program completed an experimental version of the MBS and different measures of the quality of the mentoring relationship at two times during the program. The questionnaire has good internal consistency coefficients and adequate factorial structure, with the exception of the factor autonomy support. Moreover, three dimensions of the MBS predict mentoring relationship quality and the perceived usefulness of the intervention. Recommendations for the use and improvement of the MSB are proposed.