The role of Guidance and Counselling in making informed choices among learners cannot be overemphasized. Although institutional-based Guidance and Counselling was introduced in Kenya to equip learners with the skills of making informed decisions so as to reduce risky sexual behaviours and teenage pregnancies. The numbers of teenage pregnancies are on the rise in Siaya County especially Ugenya Sub County in Kenya. The aim of this study was to assess the effectiveness of peer counselling in management of teenage pregnancy among girls. The study utilized a Concurrent Triangulation Design within the Mixed Method Approach. Rational Emotive Behaviour Therapy theory informed the study. Purposive sampling technique was used to sample 15 Principals and 15 Teacher Counsellors. Kerlinger formula was employed to determine and select 347 teenage girls in Form 2 and 3 to participate in the study, while 50 Peer Counsellors were selected to participate in the study using stratified random sampling. Data for the study was collected using questionnaires, structured interview schedules and Focus Group Discussions. Data collected from the questionnaires was edited, coded and analyzed through descriptive statistics as well as Pearson Correlation analysis using SPSS version 23. Thematic Analysis was used to analyse qualitative data. The findings indicated that there was a strong and positive correlation (r = .675) between peer counselling and the management of teenage pregnancy at p < 0.05. The Ministry of Education should emphasize more diverse training of Guidance and Counselling teachers on varied therapies for them to be effective in schools. In addition, the policy makers in education should also come up with various training programs meant for peer counsellors to assist in therapy programmes in their respective secondary schools, since students preferred seeking counselling services from their peer counsellors.