To promote improved student outcomes, school mental health providers (SMHPs) monitor implementation fidelity when implementing evidence-based interventions. Current approaches for implementation fidelity monitoring (e.g., intervention checklists) do not account for other relevant implementation indicators best captured through qualitative data. Therefore, this study employed a multi-faceted approach to monitoring implementation fidelity of the Mental Health Action Plan (MHAP) Program, a school-based person-centered care planning program developed to coordinate mental health care at school. The purpose of this mixed methods study was to explore the extent to which the MHAP Program was implemented as intended by describing determinants (i.e., barriers and facilitators) to adherent implementation. Three participant triads (student, caregiver, teacher) and three SMHPs were included in the study. Implementation fidelity checklists and field note data was collected and analyzed using reflexive thematic analysis and descriptive statistics. Qualitative results provided greater context for the quantitative results and elucidated potential adaptations to enhance youth-intervention fit. Themes related to the caregiver-child relationship, logistical barriers, student factors, SMHP competing responsibilities, and team dynamics impacted implementation fidelity. Implications for monitoring implementation fidelity of school mental health interventions via qualitative methods as well as study limitations are discussed.