Adolescent sexual behaviour in Nairobi, Kenya is a public health concern with high rates of teenage pregnancy and sexually transmitted infections (STIs). Effective interventions are needed to address these issues. A mixed-methods approach involving pre- and post-intervention surveys and focus group discussions among adolescents aged 15-18 years was used. Data were analysed using descriptive statistics and thematic analysis. Prevalence of unprotected sexual intercourse decreased by 30% (95% CI: 20%, 40%) after the health promotion campaigns, indicating a significant impact on adolescent behaviour. Health promotion campaigns significantly reduced unprotected sexual activity among adolescents in Nairobi. Further research is needed to explore long-term effects and sustainability of these interventions. Continuation and expansion of such health education programmes are recommended to maintain the reduction in risky behaviors observed after intervention. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.