A significant body of research has illustrated that the Risk/Need/Responsivity (RNR) paradigm is predictive of recidivism for a variety of offenders, including those with mental illness. This paper builds upon the extant literature in this area by examining how RNR domains specifically predict successful completion of a mental health specialty caseload. To do so, we use data collected from offenders on the Seriously Mentally Ill probation caseload in Maricopa County, Arizona to examine the completion of probation. Using the Offender Screening Tool (OST) to examine how risk/need predicts probation completion (n = 1,430), our findings demonstrate that the OST score overall, as well as participants’ age, whether the probationer received a Petition for Revocation for a technical violation or new crime during their time on probation, and multiple specific OST risk domains (e.g., educational risk, drug risk, mental health risk, and criminal behavior risk) are meaningfully associated with probation completion. We close by contextualizing these findings and discussing their policy implications.