Background: Early literacy is a critical determinant of cognitive development, academic success, and long-term socio-economic mobility. In Botswana, where linguistic diversity and resource constraints pose significant challenges, the implementation of early literacy curricula in pre-primary schools remains uneven, impacting language development outcomes. Purpose: This study investigates the implementation fidelity of Botswana’s early literacy curriculum and its effect on language development in pre-primary schools, focusing on phonological awareness, vocabulary acquisition, and reading comprehension. Design: A sequential explanatory mixed-methods design was employed, combining quantitative assessments of 320 children across 16 schools with qualitative interviews of 25 teachers and 10 policymakers. Hierarchical Linear Modelling (HLM) was used to analyse the relationship between curriculum fidelity and language gains, while thematic analysis of interviews and classroom observations provided contextual insights. Findings: Curriculum fidelity averaged 65%, with urban schools achieving 75% fidelity compared to 55% in rural schools. Higher fidelity was associated with improved literacy outcomes, particularly in vocabulary (mean gain of 16.2 points in urban schools vs. 10.4 in rural schools). Key factors influencing fidelity included teacher training (OR = 1.92, p < 0.001), resource availability (OR = 2.30, p < 0.001), and linguistic diversity (OR = 1.95, p = 0.001). Qualitative data revealed challenges such as inadequate training, material shortages, and linguistic mismatches between home and school languages. Implications: The study underscores the need for localized adaptations of the curriculum, sustained teacher training, equitable resource distribution, and flexible lang