Introduction: In 2013, the Zambian Correctional Service (ZCS) partnered with the Centre for Infectious Disease Research in Zambia on the Zambian Prisons Health System Strengthening project, seeking to tackle structural, organisational and cultural weaknesses within the prison health system. We present findings from a nested evaluation of the project impact on high, mid-level and facility-level health governance and health service arrangements in the Zambian Correctional Service . Methods: Mixed methods were used, including document review, indepth interviews with ministry (11) and prison facility (6) officials, focus group discussions (12) with male and female inmates in six of the eleven intervention prisons, and participant observation during project workshops and meetings. Ethical clearance and verbal informed consent were obtained for all activities. Analysis incorporated deductive and iterative inductive coding. Results: Outcomes: Improved knowledge of the prison health system strengthened political and bureaucratic will to materially address prison health needs. This found expression in a tripartite Memorandum of Understanding between the Ministry of Home Affairs, Ministry of Health (MOH) and Ministry of Community Development, and in the appointment of a permanent liaison between MOH and ZCS. Capacity-building workshops for ZCS Command resulted in strengthened health planning and management outcomes, including doubling ZCS health professional workforce (from 37 to78 between 2014 and 2016), new preservice basic health training for incoming ZCS officers and formation of facility-based prison health committees with a mandate for health promotion and protection. Mechanisms: continuous and facilitated communication among major stakeholders and the emergence of interorganisational trust were critical. Enabling contextual factors included a permissive political environment, a shift within ZCS from a 'punitive' to 'correctional' organisational culture, and prevailing political and public health concerns about the spread of HIV and tuberculosis. Conclusion: While not a panacea, findings demonstrate that a 'systems' approach to seemingly intractable prison health system problems yielded a number of short-term tactical and long-term strategic improvements in the Zambian setting. Context-sensitive application of such an approach to other settings may yield positive outcomes.