Abstract Background and Objectives Significant quality problems exist in long-term care (LTC). Interventions to improve care are complex and often have limited success. Implementation remains a black box. We developed a program theory explaining how implementation of a complex intervention occurs in LTC settings—examining mechanisms of impact, effects of context on implementation, and implementation outcomes such as fidelity. Research Design and Methods Concurrent process evaluation of Safer Care for Older Persons in residential Environments (SCOPE)—a frontline worker (care aide) led improvement trial in 31 Canadian LTC homes. Using a mixed-methods exploratory sequential design, qualitative data were analyzed using grounded theory to develop a conceptual model illustrating how teams implemented the intervention and how it produced change. Quantitative analyses (mixed-effects regression) tested aspects of the program theory. Results Implementation fidelity was moderate. Implementation is facilitated by (a) care aide engagement with core intervention components; (b) supportive leadership (internal facilitation) to create positive team dynamics and help negotiate competing workplace priorities; (c) shifts in care aide role perceptions and power differentials. Mixed-effects model results suggest intervention acceptability, perceived intervention benefits, and leadership support predict implementation fidelity. When leadership support is high, fidelity is high regardless of intervention acceptability or perceived benefits. Discussion and Implications Our program theory addresses important knowledge gaps regarding implementation of complex interventions in nursing homes. Results can guide scaling of complex interventions and future research.