Past research suggests that bilingualism positively affects children's performance in false belief tasks. However, researchers have yet to fully explore factors that are related to better performance in these tasks within bilingual groups. The current study includes an assessment of proficiency in both languages (which was lacking in past work) and investigates the relationship between proficiency and performance in a variety of mental state tasks (not just false belief). Furthermore, it explores whether the relationship between language proficiency and performance in mental state tasks differs between bilingual and monolingual groups. Twenty-six Spanish-English bilingual and twenty-six English monolingual preschool-age children completed seven mental state tasks. Findings provide evidence that high proficiency in English is related to better performance in mental state tasks for monolinguals. In contrast, high proficiency in both English and Spanish is related to better performance in mental state tasks for bilinguals.