Abstract Background Many people with dementia live in care homes, where staff can struggle to meet their complex needs. Successful practice improvement interventions in these settings require strong managerial support, but little is known about how managers can support implementation in practice, or what factors support or hinder care home managers in providing this support. Using Dementia Care Mapping™ (DCM) as an example, this study explored how care home managers can support the implementation of complex interventions, and identified factors affecting their ability to provide this support. Methods We undertook interviews with 48 staff members (managers and intervention leads) from care homes participating in the intervention arm of the DCM EPIC trial of DCM implementation. Results Managerial support played a key role in facilitating the implementation of a complex intervention in care home settings. Managers could provide practical and financial support in many forms. However, managerial support and leadership approaches towards implementation were highly variable in practice, and implementation was easily de-stabilised by management changes or competing managerial priorities. How well managers understood, valued and engaged with the intervention, alongside the leadership style they adopted to support implementation, were key influences on implementation success. Conclusions For care home managers to effectively support interventions they must fully understand the proposed intervention and its potential value. This is especially important during times of managerial or practice changes, when managers lack the skills required to effectively support implementation, or when the intervention is complex. It may be unfeasible to successfully implement new interventions during times of managerial or practice instability. Trial registration Current Controlled Trials ISRCTN82288852, registered 16/01/2014.