Mobile phone usage is widespread in urban slums of Nairobi, Kenya, where access to mental health services remains limited. A mixed-methods approach was employed, including pre- and post-intervention surveys, focus group discussions, and qualitative interviews to assess participants' engagement and satisfaction levels with the service. Participants reported a significant improvement in mental health scores (mean increase of 15%, CI: 9.5% - 20.5%) after participating in the mobile phone-based support groups, indicating positive outcomes. Themes emerged around the convenience and accessibility of the service among participants. The mobile phone-based support groups demonstrated effectiveness in improving mental health among urban slum populations in Nairobi, with user satisfaction rates exceeding 85%. Further studies should explore scalability and cost-effectiveness to extend this model to other urban slums. Implementation of a randomized controlled trial is recommended for validation. Mobile Phone-Based Support Groups, Mental Health, Urban Slums, Nairobi, Kenya Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.