ABSTRACT: Transition-age youth have elevated rates of mental disorders, and they often do not receive services. This is a serious public health concern, as mental health conditions persist into adulthood. Continuing to engage this population has been a pervasive challenge for the mental health care system worldwide. Few mental health interventions have been developed for transition-age youth, and even fewer have been found to be effective over the transition to adulthood. Cornerstone, a theoretically guided intervention has shown promise for addressing the mental health and psychosocial needs of this population as they emerge into adulthood. Cornerstone provides case management, trauma-focused cognitive behavioral therapy, mentoring/peer support, community-based in vivo practice, and groups to address stigma, mistrust, and practical skill development to improve the transition to independence among transition-age youth with serious mental health conditions.This study utilizes a hybrid research design and focuses on examining feasibility, acceptability and preliminary impact, along with factors that influence implementation, to maximize new knowledge. The study combines qualitative methods and a randomized controlled trial, using data to inform and refine protocols and manuals, while testing the preliminary impact of the intervention, compared to best available services (treatment as usual, TAU) at a partnering outpatient mental health clinic (n?=?60). Contributors to the intervention development research (n?=?20) are national experts on mental health services, clinic administrators and staff and young adults with direct experience. The intervention involves intensive staff training and 18 months of ongoing service provision, monitoring and supervision. Quantitative survey data will be collected at baseline, 3 months, 6 months, and 9 months measuring mental health and practical life outcomes via self-report measures. Medical records will be used to triangulate self-report data (i.e., primary diagnosis, treatment planning and attendance). Qualitative data focuses on the intervention development process and implementation research and will use constant comparison coding techniques. In this intention-to-treat analysis, we will conduct basic omnibus analyses to examine whether Cornerstone leads to improved outcomes relative to TAU utilizing t tests across treatment conditions for each outcome measure specified. We will likewise examine whether changes in the proposed mediating variables differ across groups.The aim of this study is to refine Cornerstone through an intensive preliminary trial, learning through collaboration with clinic staff, project team members, and leaders in New York State and nationwide on how to best serve transition-age youth with serious mental health conditions. Cornerstone has the potential to fill a large gap in the service system for transition-age youth with serious mental health conditions, and may enhance the menu of care options for those who have been recently diagnosed with a serious mental health condition, and yet, have a long life to live. The program is recovery-oriented, builds on the best evidence to date, and is in line with both local and national health care reform efforts.This trial was registered with ClinicalTrials.gov (Identifier: NCT02696109 ) on 22 April 16 as Protocol Record R34-MH102525-01A1MRM, as New York University, Cornerstone program for transition-age youth with serious mental illness: study protocol for a randomized controlled trial.