The widespread integration of smartphones into adolescent life has raised growing concern regarding their mental health implications. This cross-sectional study surveyed 1,200 adolescents aged 13–18 across three urban districts in Accra, Ghana, assessing screen time, sleep duration, and reported anxiety and depressive symptoms. Multivariate regression revealed a strong association between daily smartphone use exceeding five hours and elevated anxiety scores (p < 0.01). Sleep disruption mediated 34% of this relationship. While digital connectivity offers social benefits, overuse correlates with adverse psychological outcomes. Public health interventions must balance technology access with mental well-being education and screen-time hygiene programs for youth.