Adolescents in Nairobi slums face significant challenges related to HIV infection and reproductive health outcomes. A mixed-methods approach including surveys and qualitative interviews was employed among participants aged 10-19 years in Nairobi slums. The analysis revealed a reduction in unprotected sexual intercourse from 45% to 32%, with statistical significance at p < 0.05, indicating a preventive effect of the programme (OR = 0.67; CI: 0.49-0.89). The school-based HIV prevention programmes demonstrated a positive impact on adolescent reproductive health in Nairobi slums. Continuation and expansion of these programmes are recommended to further reduce the risk factors for HIV infection among adolescents.