<h3>Introduction</h3> In 2018, 1.6 million fee-for-service (FFS) Medicare beneficiaries used skilled nursing facilities (SNFs) at least once. Older adult SNF patients with mental illness (MI) and Alzheimer's disease and related dementias (ADRD) comprise groups that are at risk of nursing home long-term care. We examined how the presence of MI and ADRD were associated with discharge from SNF admissions to the community and home time (the number of days alive that were not in a hospital, inpatient rehabilitation facility, or SNF). We hypothesized that older adults with MI and ADRD were less likely to have been discharged to the community and had less home time. We anticipated that these associations would be attenuated, however, after accounting for the behavioral and psychological manifestations of MI and ADRD such as daily functioning, depressive symptoms, and aggressive behaviors. <h3>Methods</h3> Our study examined 46,342 New York State FFS Medicare beneficiaries aged 65 years and older with an index SNF admission in 2014 following a hospitalization of three or more days. Our first outcome was whether an SNF patient was discharged to the community within 100 days of SNF admission (binary: yes/no). Included in the "no" category were older adults who were discharged from the SNF to a hospital or a different SNF, remained in the SNF longer than 100 days, or died. For those discharged to the community, our second outcome was home time (continuous) in the 90 days following SNF discharge. The key independent variable was the presence of a MI or ADRD diagnosis, which we organized into three groupings: 1) MI present and ADRD absent, 2) ADRD present (with and without MI), and 3) both MI and ADRD absent (serving as reference group). We extracted data from the Minimum Data Set, Medicare claims, and other administrative databases. We conducted logistic and zero-inflated negative binomial regressions to analyze the associations between MI/ADRD grouping with our outcomes. <h3>Results</h3> Among SNF post-acute care admissions, 23.0% had MI only, 22.7% had ADRD, and 58.9% were discharged to the community. In analyses adjusting for socio-demographic and clinical characteristics, MI and ADRD were associated with decreased odds of community discharge (MI: odds ratio, OR=0.90, 95% confidence interval, CI: 0.86-0.95; ADRD: OR=0.51, 95% CI: 0.49-0.54). When the manifestations of MI and ADRD were added to the regression models, however, MI was no longer associated with community discharge and ADRD's association was attenuated (MI: OR=0.97, 95% CI: 0.92-1.02; ADRD: OR=0.71, 95% CI: 0.67-0.75). Among those discharged to the community, after adjusting for socio-demographic and clinical characteristics, MI and ADRD were associated with 1.26 and 1.41 fewer days in the community (MI: -1.26 days, 95% CI: -1.95, -0.57; ADRD: -1.41 days, 95% CI: -2.21, -0.60). When the manifestations of MI and ADRD were added to the regression models, MI's association with home time persisted, but ADRD was no longer associated (MI: -0.73 days, 95% CI: -1.41, -0.05; ADRD: 0.45 days, 95% CI: -0.27, 1.17). <h3>Conclusions</h3> Among NYS FFS Medicare beneficiaries who received post-acute care SNF services, older adults with MI and ADRD had worse SNF discharge outcomes. These worse outcomes appear to have been at least partially driven by impaired daily functioning, depressive symptoms, and aggressive behaviors that can be consequences of MI or ADRD. Accordingly, psychogeriatric care to optimally manage the symptoms and behaviors associated with MI and ADRD may be necessary to improve community discharge and home time. <h3>Funding</h3> Dr. Simning was supported by the Empire Clinical Research Investigator Program, sponsored by the New York State Department of Health, as well as the National Institute on Aging (grant number K23AG058757). The content is solely the responsibility of the authors and does not necessarily reflect the official views of the National Institutes of Health.