Refugee newcomers resettled in the United States face numerous challenges impacting their mental health and overall well-being. Despite extensive research on clinical interventions, multimodal interventions with peer-based group models in resettlement contexts remain scarce. Adopting a realist evaluation approach, this study aims to conduct a comprehensive implementation evaluation of a complex community-based mental health and psychosocial support (CB-MHPSS) group intervention, examining its mechanisms and processes while considering the interplay among context, implementing agents, and community settings. Qualitative and quantitative data on the implementation process were collected from 11 refugee agencies, involving trained resettlement staffers (n = 32) and refugee community facilitators (n = 31) who implemented the 31 CB-MHPSS psychosocial groups in 2021. The analysis included fidelity reports, process reflections, and follow-up survey responses, utilizing a structured template based on the CB-MHPSS Theory of Change (ToC) and Medical Research Council (MRC) guidance. Results of an in-depth exploration of complex relations between context and implementation processes reveal the vital role that adaptability to socio-ecological circumstances during the global coronavirus pandemic played in achieving successful implementation of psychosocial group interventions. Striking a balance between fidelity and refinement of intervention of the intervention curriculum emerged as crucial factors for aligning facilitation goals with newcomer needs. This approach maintained the integrity and appropriateness of the intervention, requiring implementers to utilize local knowledge of community resources and their understanding of lived experience of forced migrants and their within the community. Furthermore, exploring intervention barriers and facilitators revealed merits aligned with program change theories and areas for adjustment, while cataloged guidelines addressed community needs, showcasing competence beyond standard checkboxes. Strategic dissemination guidance provided by the central office in a supportive and nonintrusive manner facilitated uptake in a participatory and context-specific manner. This study emphasizes the importance of leveraging community partnerships and local knowledge to result in sustained improvements in the mental health and well-being of refugees.