Background A Dutch committee for National Guidelines in Neonatology developed nineteen evidence- and consensus-based guidelines to be used in all Dutch neonatal intensive care units (NICUs). The primary goal was to make clinical practices more uniform and consistent. Objective This study investigated to what extent the guidelines were implemented and which factors played a role in implementation. Study design A mixed method study design was used to investigate both the level and the process of implementation. A nationwide, multicenter, cross-sectional survey was performed using a validated instrument for measuring the level of implementation (Normalization MeAsure Development questionnaire, NoMAD). The number of implemented guidelines per NICU and the frequency and content of the amendments that NICUs made to the original consensus guidelines were analyzed. Through semi-structured interviews, perceived barriers and facilitators for implementation were explored. Participants Fellows and neonatologists working at all ten Dutch level 3-4 NICUs were eligible. Results On an average, NICUs implemented 12.6 (of 19) guidelines (range 6-17). The Normalization Process Scale was 54 (of 65). Main influencing factors impeding implementation were guideline-related (e.g., unpractical, lengthy guidelines) and personal (e.g., an active representative responsible for local implementation). Conclusion The implementation of our guidelines appears to be successful. Ways for improvement can be distinguished in personal, guideline-related and external factors. Empowerment of local representatives was considered most essential.