Providing instruction in a student's home language through bilingual education has proven to be academically beneficial for multilingual learners (MLLs). The present study contributes to this literature by examining the possibilities and limits of a professional development program (Mathematics and Language Literacy Integration [MALLI] in Dual Language settings) with dual language (Spanish/English) teachers and the impact on elementary grade MLLs' state mathematics and English language proficiency assessment outcomes. MALLI was a year-long professional development project that included models for integrating language and literacy into Spanish-language mathematics instruction using Lesson Study with Video Clubs. Coarsened exact matching was utilized to establish a treatment and unbiased comparison group. MLLs taught by MALLI teachers had state mathematics assessment average standardized scores of 0.374 standard deviation units higher than comparison students whose teachers did not participate in MALLI. These results were statistically significant, equivalent to moving the performance of an average MLL from the 50th percentile to the 64.6 percentile rank. However, the effect on State English language proficiency scores was small and not statistically significant. This study illustrates the potential impact of specially designed professional development to improve MLLs' mathematics outcomes.