<p>This longitudinal study examines the developmental relationships between language switching ability and executive function components in English learners across multiple proficiency levels. Drawing on bilingual cognitive control theory, we investigated how working memory, inhibitory control, and cognitive flexibility interact with language switching performance over an 18-month period. Participants (<i>N</i> = 266) completed comprehensive assessments of language switching tasks, executive function measures, and English proficiency tests at six time points. Results revealed systematic improvements in both language switching efficiency and executive function performance, with switching costs and mixing costs showing significant reductions over time. Growth curve modeling demonstrated bidirectional relationships between executive function components and language switching ability, with inhibitory control and cognitive flexibility emerging as stronger predictors than working memory. Individual differences in developmental trajectories were significantly moderated by English proficiency level, age, and baseline executive function capabilities. Intermediate proficiency learners exhibited the steepest improvement curves, suggesting an optimal developmental window for cognitive control enhancement. These findings provide empirical support for theoretical models proposing that language switching practice strengthens domain-general executive control systems while executive function capabilities facilitate more efficient bilingual language management.</p>