Young adults experiencing homelessness (YAEH) have high prevalence of mental disorders, however, they face unique access barriers to mental health services combined with low recognition of mental health symptoms, leading to low rates of mental health service utilization. The point of transition from homelessness to housing presents a period of opportunity to identify mental health symptoms and connect and engage YAEH into mental health services to reduce symptoms and promote housing stability. The COVID-19 pandemic has heightened mental health symptoms and increased the need for services among YAEH, but it has also resulted in recovery funds to increase housing supports in many communities. There is a critical need to develop interventions that can support YAEH with mental health challenges as they make the transition from homelessness to housing as more supported housing becomes available. Critical Time Intervention (CTI), a structured, time limited case management intervention has demonstrated success with adults with serious mental illness in multiple randomized controlled trials but it has not been tested in YAEH. The goal of this study is to adapt CTI for the context of transition from homelessness to supported housing within the rapid rehousing program, integrating mental health specific content from a young adult treatment model, C4, to develop CTI-YAMH (young adult mental health), and then test the new intervention in a feasibility pilot. Specifically, we aim to: Aim 1: Refine the draft CTI- YAMH intervention (treatment, training and assessment protocols) to ensure the target mechanisms are adequately addressed for stabilizing housing and mental health, utilizing an iterative stakeholder feedback process to finalize the manuals for pilot testing, then Aim 2: Conduct an open trial of the adapted CTI- YAMH intervention to assess the feasibility of randomization procedures, refine outcome measures, assess acceptability, and examine the preliminary signal of impact of the intervention. This innovative study targets a critical point of intervention, the transition from homelessness to housing, for an extremely marginalized group (YAEH), utilizing an innovative adaptation framework, ADAPT-ITT, to systematically adapt CTI in partnership with youth with lived expertise and community providers. The CTI-YAMH intervention aims to support a population with high unmet need for mental health services through a model that can be paired with rapid rehousing, a supportive housing model widely used in communities across the U.S. Results from this R34 study lay the foundation for a fully powered RCT of the CTI-YAMH intervention in a future R01 study.