The objective of implementing evidence-based practices in health services is to facilitate quality care. Participatory action research (PAR) is a qualitative method that has been used to implement such changes in clinical practice. PAR is characterized by the participation, involvement, and commitment of the participants to improve a specific phenomenon of collective interest. Throughout this case study, the most relevant aspects of the process of implementing evidence-based practices in a mental health unit are described and reflected upon. The study was conducted with mixed methods. First, PAR was conducted with mental health nurses to establish and implement evidence-related changes in their clinical practice. For this method, data collection techniques were used in group sessions and through individual diaries. Second, the impact of changes in the level of the nurses’ therapeutic relationship with patients was quantitatively evaluated through pre- and post-PAR measures. The main limitations that appeared throughout the project were the excessive duration of the process and the lack of flexibility in the collection of data through journals, whereas the main strengths of the project were the ability to integrate the generation of knowledge and direct application in practice, the relevance of the topic of study for the participants, and the constant questioning in the participants and researchers during the process. This study illustrates the importance of the central theme of the study, the role of researchers and participants, and the adequacy of data collection techniques. With this case study, readers can learn how mixed methods were integrated into the design and, specifically, the importance of the central theme of the study, the role of researchers and participants, and the adequacy of data collection techniques for conducting PAR.