Individuals with mental illness are significantly overrepresented in the Canadian justice system. Given the high rate of mental illness among individuals who are incarcerated, correctional facilities must implement accessible and effective mental health resources. This not only improves their health and well-being but also contributes to their rehabilitation efforts. However, evidence suggests that the care provided in prisons is inadequate. This scoping review asks, “What is known about the access and quality of mental health care services for adults who are incarcerated in Ontario?” Mental health care services included non-acute interventions and care that is provided in the institution. This scoping review followed the PRISMA Extension for Scoping Reviews methodology. Databases searched include MedLINE, EMBASE, CINAHL, PsycINFO, Criminal Justice Abstracts, JSTOR, Google Scholar, and the grey literature. The search yielded 354 titles and abstracts of which 16 met the inclusion criteria. Conducted from 2010-2022, the 16 studies included qualitative, quantitative, and mixed methods. Common themes that were identified related to segregation, mental health assessments, medication prescribing and access, opioid agonist therapy, psychiatric service access, systemic and institutional barriers, mental health perception, and the need for collaboration. Despite the significant demand for mental health care in Ontario correctional facilities, limitations to quality care are evident. Such limitations intersect and are then exacerbated, resulting in poor mental health care provision among the incarcerated population. More research is warranted regarding the access, quality, and efficiency of mental health care in Ontario prisons, and how factors including ethnicity, gender, and prison classification (provincial vs. federal) may influence mental health care and its outcomes.