Mentoring is a common form of support for beginning teachers. State and district mentoring policies vary along a number of dimensions, yet policymakers have little evidence to draw on in designing effective mentoring programs. We use quantitative and qualitative data from a study of beginning middle school mathematics teachers in 10 districts to investigate the relationship of mentor policy features with the quality of mentoring received by new teachers. We find a strong and significant association indicating that mentors who have time during the day to meet with their new teachers are associated with higher-quality mentoring interactions. These results are supported through teacher interviews, and interviews also suggest that teachers experience better outcomes when mentors are located in the same school and have an evaluative role. These results suggest that proximity and availability may be particularly important features if mentors are to provide high-quality support to new teachers.