This study investigates if participation in mother tongue instruction (henceforth MTI) impacts the biliteracy proficiency of young bilinguals, drawing on examples from Somali–Swedish bilinguals and Somali MTI in a Swedish school context. In the study, biliteracy was operationalized as reading proficiency and vocabulary knowledge in two languages, which was tested with measures of word decoding, reading comprehension, and vocabulary breadth and depth. The study was designed to allow for cross-sectional, longitudinal, and cross-linguistic analyses of data. Overall, the results showed that participation in MTI contributed positively to participants’ results on Somali reading comprehension, beyond the influence of chronological age, age of arrival, and reported home language and literacy use. Furthermore, higher results in Somali were associated with higher results on the same measures in Swedish, in particular for the reading measures. In sum, the results indicate that MTI has an impact on some aspects of literacy proficiency in the mother tongue, despite the restricted time allocated for it (<1 h/week). They also indicate that MTI, albeit indirectly, may benefit the stated proficiencies in the language of schooling.