Despite the prevalence of mentally ill probationers, and probation officers’ (POs’) central role in their supervision, this is the first reported study to investigate how POs implement mandates to participate in psychiatric treatment. Five focus groups were conducted in major cities with 32 POs and 20 probationers representing a mix of traditional and “specialty” probation agencies. Three key findings resulted. First, there were considerable differences between POs in specialty and traditional agencies in the nature, range, and timing of strategies applied to monitor and enforce treatment compliance. Second, the quality of PO-probationer relationships colored POs’ use of these strategies and was perceived as central to probationer outcomes. Relationships characterized by a respectful, personal, approach were perceived as more effective in achieving desired outcomes than those that were more authoritarian. Third, specialty agencies strongly emphasized offender rehabilitation whereas traditional agencies focused more exclusively on community safety. These agencies differed in how well probationers with mental illness “fit” their standard operating procedure. Implications for future research and directions for probation practice are discussed.