The main aim of this study was to evaluate the effectiveness of Pennsylvania’s state-wide intensive in-home treatment for youth with serious emotional disturbance (SED), EcoSystemic Structural Family Therapy-Family Based Mental Health Services (ESFT-FBMHS). Despite its long history of implementation, the program remains empirically under-evaluated. In this archival study, out-of-home placement and youth functioning outcomes were compared across four tiers of length of stay. Given the high-risk population treated in ESFT-FBMHS, it was hypothesized that the families and youth who completed the full duration of treatment (169–224 days) would have better outcomes than those who stopped treatment after 168 days or less. We utilized an ex post facto quantitative research design analyzing archived medical claims data of 2251 youth treated between 2018 and 2022 to assess out-of-home placement rates and analyzed archived data from six domains of the Child and Adolescent Needs and Strengths (CANS; Problem Presentation, Risk Behaviors, Functioning, Child Safety, Caregiver Needs, and Child Strengths) to assess changes in youth functioning post-discharge (90 and 180 days). An analysis using generalized estimating equations (GEE), controlling for potential confounding variables such as demographics and clinical features, suggest that length of stay in ESFT-FBMHS was significantly associated with out-of-home placement and youth improvement on the CANS at both 90- and 180-days post-discharge. As a group, youth with SED who did not complete the full duration of the program had 2–3 times the odds of out-of-home placement at 90 days post-discharge and 1–3 times the odds at 180 days post-discharge as compared to program completers. CANS scores showed improvement in 40.1% of youth who completed the program as compared to only 11.7%–18.2% for those who did not. The results of this study suggest that ESFT-FBMHS is effective for youth with SED as a group and can improve youth functioning and reduce out-of-home placement.