BackgroundImplementation science seeks to systematically identify determinants, strategies, and outcomes within a causal pathway to help explain successful implementation. This process is applied to evidence-based interventions (EBIs) to improve their adoption, implementation, and sustainment. However, this method has not been applied to exercise oncology services, meaning we lack knowledge about implementing exercise EBIs in routine practice. This study aimed to develop causal pathways from the determinants, strategies (including mechanism of change), and implementation outcomes to explain exercise EBIs implementation in routine cancer care.MethodsA multiple-case study was conducted across three healthcare sites in Australia. Sites selected had implemented exercise within routine care for people diagnosed with cancer and sustained the delivery of services for at least 12 months. Four data sources informed the study: semi-structured interviews with staff, document reviews, observations, and the Program Sustainability Assessment Tool (survey). Framework analysis was applied to understand the findings. The Implementation Research Logic Model was used to identify commonalities in implementation across sites and develop causal pathways.ResultsTwo hundred and eighteen data points informed our findings. Across sites, 18 determinants and 22 implementation strategies were consistent. Sixteen determinants and 24 implementation strategies differed across sites and results of implementation outcomes varied. We identified 11 common pathways that when combined, help explain implementation processes. The mechanisms of implementation strategies operating within the pathways included (1) knowledge, (2) skills, (3) secure resources, (4) optimism, and (5) simplified decision-making processes associated with exercise; (6) relationships (social and professional) and support for the workforce; (7) reinforcing positive outcomes; (8) capability to action plan through evaluations and (9) interactive learning; (10) aligned goals between the organisation and the EBI; and (11) consumer-responsiveness.ConclusionThis study developed causal pathways that explain the how and why of successful implementation of exercise EBIs in cancer care. These findings can support future planning and optimisation activities by creating more opportunities for people with cancer to access evidence-based exercise oncology services.Implications for cancer survivorsUnderstanding how to implement exercise within routine cancer care successfully is important so cancer survivors can experience the benefits of exercise.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00520-023-07844-x.