BackgroundAlthough there are many proven effective physical activity (PA) interventions for older adults, implementation in a real world setting is often limited. This study describes the systematic development of a multifaceted implementation intervention targeting the implementation of an evidence-based computer-tailored PA intervention and evaluates its use and feasibility.MethodsThe implementation intervention was developed following the Intervention Mapping (IM) protocol, supplemented with insights from implementation science literature. The implementation intervention targets the municipal healthcare policy advisors, as an important implementation stakeholder in Dutch healthcare system. The feasibility of the implementation intervention was studied among these stakeholders using a pretest–posttest design within 8 municipal healthcare settings. Quantitative questionnaires were used to assess task performance (i.e. achievement of performance objectives), and utilization of implementation strategies (as part of the intervention). Furthermore, changes in implementation determinants were studied by gathering quantitative data before, during and after applying the implementation intervention within a one-year period. Additionally, semi-structured interviews with stakeholders assessed their considerations regarding the feasibility of the implementation intervention.ResultsA multi-faceted implementation intervention was developed in which implementation strategies (e.g. funding, educational materials, meetings, building a coalition) were selected to target the most relevant identified implementation determinants. Most implementation strategies were used as intended. Execution of performance objectives for adoption and implementation was relatively high (75–100%). Maintenance objectives were executed to a lesser degree (13–63%). No positive changes in implementation determinants were found. None of the stakeholders decided to continue implementation of the PA intervention further, mainly due to the unforeseen amount of labour and the disappointing reach of end-users.ConclusionThe current study highlights the importance of a thorough feasibility study in addition to the use of IM. Although feasibility results may have demonstrated that stakeholders broadly accepted the implementation intervention, implementation determinants did not change favorably, and stakeholders had no plans to continue the PA intervention. Yet, choices made during the development of the implementation intervention (i.e. the operationalization of Implementation Mapping) might not have been optimal. The current study describes important lessons learned when developing an implementation intervention, and provides recommendations for developers of future implementation interventions.