INTRODUCTIONSmoke-free workplaces are considered an important part of tobacco control strategies. The purpose of this study was to evaluate implementation fidelity and explore the significance of social and contextual factors for the implementation of a strict smoke-free workplace intervention in a large Danish medical company.METHODSThe UK Medical Research Council’s guidance for process evaluation was used as a framework. Data were collected from approximately six months before the implementation until ten months after (2019–2020). A mixed method study design was used (a survey of 398 employees, a focus group of four employees and field visits on two days). Data were analyzed separately and later integrated through triangulation. We used the Fisher’s exact test in the analysis of the questionnaire.RESULTSWe assessed the implementation fidelity through four key factors: reach, dose and delivery, mechanisms of change, and context for the intervention components. Despite compliance issues, the policy component had high implementation fidelity. However, the implementation fidelity of the smoking cessation support component was low. We identified three social mechanisms influencing the employees’ responsiveness towards the policy: expectation, the social aspect of the smoking facilities, and management leadership. COVID-19 was identified as the main contextual factor affecting the implementation.CONCLUSIONSAlthough not all elements of the intervention components were implemented as planned, the strict smoke-free workplace intervention is considered implemented. Further strategies can be initiated to raise implementation fidelity through better communication concerning the cessation support component, compliance, and enforcement of the policy.