Pediatric hematological and oncological illnesses present many coping challenges. Mental health issues can arise during and after treatment, in both patients and their families. The current model of care does not always seem to meet the needs identified by some young patients. In Quebec, nurses are allowed to assess and care for patients experiencing physical and mental health difficulties (Ordre des infirmières et infirmiers du Québec, 2016). Therefore, a mental health nurse clinician (MHNC) with experience in pediatric hematology/oncology could provide care that is complementary to that offered by psychologists, social workers, and other psychosocial professionals in the pediatric hematology/oncology unit in order to meet any needs that remain unmet. The MHNC project has three stages: (1) role development, (2) role implementation, and (3) role assessment one year after implementation. In this first article, we explain how the MHNC role was developed. We used the participatory, evidence-based, patient-focused process for advanced practice nursing (APN) role development, implementation, and evaluation (PEPPA Framework; Bryant-Lukosius & Dicenso, 2004) to develop this model. The first five steps in the PEPPA Framework were applied in the creation of the MHNC role to (1) select the target population, (2) identify the stakeholders to be involved, (3) ascertain needs, (4) determine and prioritize problems and set goals, and (5) define a new model of care. After multiple meetings involving numerous health professionals and managers, the MHNC role was developed with a versatile, transdisciplinary perspective to address better the needs of young cancer patients (especially those in their teens) and their families. The role was developed around four main areas of practice: (1) interventions offered to patient-family, (2) interventions offered to health professionals, (3) psychiatric consultation-liaison, and (4) education and research. The next steps are to use a strategic plan to implement the role and then to evaluate the impact of the role one year after implementation.