Abstract: Aim: To evaluate whether a two‐part culture improvement programme aimed at nurses in clinical and managerial positions in an inpatient mental health service was associated with culture change, and safety‐related behaviour and knowledge improvements. Background: Due to serious failings in the delivery of physiological care to mentally disordered inpatients, it was deemed important that interventions be applied to improve service culture. Methods: A pre‐test and post‐test study was conducted to evaluate change associated with a mandated intervention aimed at culture change. Nurses in clinical and managerial positions at all levels attended relevant sessions. All were invited to participate in evaluation measures. Results: N = 241 nurses participated in the evaluation (n = 137 and n = 104, pre‐test and post‐test, respectively). There was a small but significant change in organisational culture indicating greater adhocracy and less clan culture in the second survey period and a small decline in reported safety behaviour. Measures of safety culture, knowledge and emergency‐related educational satisfaction were unchanged. Conclusion: Only a small change in measured culture was associated with the programme. Implications for Nursing Management: Attempts to evaluate culture change need to align anticipated outcomes with appropriate outcome measures. A mandated programme of culture change had little tangible effect on the outcomes measured.