Among people living with HIV (PLWH), 50% report substance use disorders (SUDs), and 30-61% report posttraumatic stress disorder (PTSD). Comorbid PTSD/SUD/HIV has been linked to faster HIV progression and twice the rate of death, lower medical adherence and retention, and increased viral load compared to PLWH without co-occurring PTSD or SUD. A critical first step in establishing comprehensive mental health services for PLWH is the implementation of an evidence-based screening protocol for PTSD and SUDs to facilitate referrals to specialty mental health providers. Guided by the Consolidated Framework for Implementation Research, this mixed-methods study aimed to examine the feasibility of delivering the REACH (Resiliency, Engagement, and Accessibility for Comorbid HIV/PTSD/SUD populations) Screening protocol. Three case managers were instructed to provide the REACH screening electronically to all patients that they saw for enrollment or re-enrollment appointments over 3 months (n=102). Of the 70 patients who completed the screener, 27% had clinically significant PTSD symptoms and 48.6% had SUD concerns. Qualitative feedback revealed themes related to beliefs about SU and PTSD, attitudes toward screening, comfort in the discussion of SU and PTSD, and referral and treatment considerations. Discussion includes lessons learned for implementation of this assessment of PTSD/SUD in PLWH as a novel approach to increase mental health engagement and promote health equity, with the potential long-term impact of improving HIV care outcomes via ameliorating mental health/SUD, and implications for prevention of HIV transmission. Implementation science can be leveraged to understand the gap in the utilization of existing evidence-based screening tools in HIV care settings.