Increasingly, states and teacher education programs are establishing minimum requirements for cooperating teachers' (CTs') years of experience or tenure. Undergirding these policies is an assumption that to effectively mentor preservice teachers (PSTs), CTs must themselves be instructionally effective. We test this assumption using statewide administrative data on nearly 2,900 PSTs mentored by over 3,200 CTs. We find the first evidence, of which we are aware, that PSTs are more instructionally effective when they learn to teach with CTs who are more instructionally effective. Specifically, when their CTs received higher observational ratings and value-added to students' achievement measures (VAMs), PSTs also received higher observational ratings and VAM during their first years of teaching; CTs' years of teaching experience, though, were mostly unrelated to these outcomes. These findings have implications for teacher education program leaders and policymakers who seek to recruit and set requirements for CTs who are more likely to support PSTs' future instructional effectiveness.