Using a mixed-methods randomised experimental design, the present study was conducted to evaluate the effectiveness of the ‘Together 4 All’ (T4A) programme in Northern Ireland. T4A was designed as a culturally adaptive version of the widely used Promoting Alternative Thinking Strategies (PATHS) programme. Its purposes are to foster pro-social and positive social-emotional behaviour among children of different religions and cultural backgrounds. Interview, observation, and assessment data were collected from children, teachers, principals, and parents from six T4A and six control schools. Year 1 results indicated: (a) implementation was adequate to strong in most classes; (b) teachers, pupils, and parents reacted positively toward the curriculum; and (c) programme effects on social-emotional learning were weak and inconsistent, but generally in a positive direction, particularly in pupils’ knowledge/expression of feelings and pro-social behaviour. Implications for programme development and sustainability are examined in light of the findings.