Purpose: The purpose of this study is to provide an example on how implementation of a new practice in mental healthcare can be carried out. We describe experiences with systematic implementation of treatment methods for post-traumatic stress disorder (PTSD) in outpatient mental healthcare clinics in Nord-Trøndelag Health Trust (HNT) in 2020–2021. We also examined how many patients were diagnosed with PTSD, and how many consultations were given targeting PTSD before, during, and after the implementation-period (2017–2023). Method: The authors’ subjective experiences with the implementation process were shared and discussed. Two of the authors were implementation facilitators, and one author was a first level leader in one of the outpatient clinics. The administrative data from the Nord-Trøndelag Health Trust were collected and analyzed. Results: The first level leaders at the four outpatient clinics initiated regular digital meetings where they shared experiences and solved problems during the implementation. All employees were involved in the implementation process. There was a larger percentage of patients with the PTSD diagnosis during the implementation period (12 %) compared to the period before (8 %) and after (10 %) the implementation. It was the same for the percentage of consultations for the same patient group (13 %) compared to the period before (9 %) and after (12 %) the implementation. Implications: This study gives a practical example of systematic implementation of a new practice in one of the Norwegian Health Trusts. The study also shows how administrative data can be used in the evaluation of an implementation. Keywords: implementation, PTSD, evidence-based practice, trauma, outpatient mental healthcare