Abstract Background Investigations of implementation factors (e.g., collegial support and sense of coherence) are recommended to better understand and address inadequate implementation outcomes. Little is known about the relationship between implementation factors and outcomes, especially in later phases of an implementation effort. The aims of this study were to assess the association between implementation success (measured by programme fidelity) and care providers’ perceptions of implementation factors during an implementation process and to investigate whether these perceptions are affected by systematic implementation support. Methods Using a cluster-randomized design, mental health clinics were drawn to receive implementation support for one (intervention) and not for another (control) of four evidence-based practices. Programme fidelity and care providers’ perceptions (Implementation Process Assessment Tool questionnaire) were scored for both intervention and control groups at baseline, 6-, 12- and 18-months. Associations and group differences were tested by means of descriptive statistics (mean, standard deviation and confidence interval) and linear mixed effect analysis. Results Including 33 mental health centres or wards, we found care providers’ perceptions of a set of implementation factors to be associated with fidelity but not at baseline. After 18 months of implementation effort, fidelity and care providers’ perceptions were strongly correlated (B (95% CI) = .7 (.2, 1.1), p = .004). Care providers perceived implementation factors more positively when implementation support was provided than when it was not (t (140) = 2.22, p = .028). Conclusions Implementation support can facilitate positive perceptions among care providers, which is associated with higher programme fidelity. To improve implementation success, we should pay more attention to how care providers constantly perceive implementation factors during all phases of the implementation effort. Further research is needed to investigate the validity of our findings in other settings and to improve our understanding of ongoing decision-making among care providers, i.e., the mechanisms of sustaining the high fidelity of recommended practices. Trial registration ClinicalTrials.gov Identifier: NCT03271242 (registration date: 05.09.2017).