The purpose of this study was to examine the effects of Mentoring Mathematical Minds (M3) units on gains in mathematics achievement and English proficiency of mathematically promising English language learners (MPELLs) in the third grade. The M3 program, developed by Gavin et al., was implemented for 1 year with treatment MPELLs (n = 86) but not with comparison MPELLs (n = 85). Treatment teachers attended 5 days of professional development provided by the program development team members. Researchers analyzed findings using hierarchical linear modeling and found that treatment students demonstrated significantly higher gains in mathematics achievement than comparison students (d = 0.63), but not in English proficiency. Qualitative data focused on treatment teachers' instructional behaviors were collected and included in considerations for high- and low-fidelity implementation with treatment students and teachers. The results supplemented the evidence that students' achievement gains in mathematics could be from the challenging mathematics content and instructional and language scaffolding strategies within the M3 units.