Youth experiencing homelessness (YEH) face complex, multilevel challenges. Existing assessment tools often overlook the unique needs of YEH population, particularly at interpersonal and structural levels. Guided by the social determinants of health (SDoH) framework and a community-based participatory approach, this study developed a comprehensive assessment tool tailored to YEH. Using a three-phase process—scoping review, expert consultation, and cognitive interviews—we identified and refined 206 items across 4 domains: individual, interpersonal, structural, and demographic. Insights from YEH and service providers revealed critical but underexplored areas, including stigma in healthcare, survival strategies, and resource-seeking behaviors for both YEH and their pets. By centering lived experience, the tool improves relevance, cultural sensitivity, and practical application. It supports individualized care planning while informing systemic interventions and policies that address root causes of homelessness. This study highlights the importance of participatory, equity-driven methods in developing meaningful tools for vulnerable young populations.