Adolescent girls in urban Kenya are at risk of substance abuse, highlighting a critical need for effective prevention programmes. A mixed-methods approach was employed, including pre- and post-intervention surveys with a sample size of 300 girls from 10 schools. Qualitative interviews were conducted to explore programme perceptions and impacts. Prevalence of substance abuse decreased by 25% among participants after the intervention (95% CI: -20% to -30%), with significant reductions in perceived stress levels, a key predictor of substance use. The school-based mental health programme was effective in reducing substance abuse and associated risk factors among adolescent girls in urban Kenya. Further research should focus on sustainability and scalability of the intervention across diverse settings. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.