Objective: We aimed to evaluate the prevalence, clinical determinants, and consequences (falls and hospitalization) of frailty in older adults with mental illness. Design: Retrospective clinical cohort study. Setting: We collected the data in a specialized psychogeriatric ward, in Boston, USA, between July 2018 and June 2019. Participants: Two hundred and fourty-four inpatients aged 65 years old and over. Measurements: Psychiatric diagnosis was based on a multi-professional consensus meeting according to DSM-5 criteria. Frailty was assessed according to two common instruments, that is, the FRAIL questionnaire and the deficit accumulation model (aka Frailty Index [FI]). Multiple linear regression analyses were conducted to evaluate the association between frailty and sample demographics (age, female sex, and non-Caucasian ethnicity) and clinical characteristics (dementia, number of clinical diseases, current infection, number of psychotropic, and non-psychotropic medications in use). Multiple regression between frailty assessments and either falls or number of hospital admissions in the last 6 and 12 months, respectively, were analyzed and adjusted for covariates. Results: Prevalence of frailty was high, that is, 83.6% according to the FI and 55.3% according to the FRAIL questionnaire. Age, the number of clinical (somatic) diseases, and the number of non-psychotropic medications were independently associated with frailty identified by the FRAIL. Dementia, current infection, the number of clinical (somatic) diseases, and the number of non-psychotropic medications were independently associated with frailty according to the FI. Falls were significantly associated with both frailty instruments. However, we found only a significant association for the number of hospital admissions with the FI. Conclusion: Frailty is highly prevalent among geriatric psychiatry inpatients. The FRAIL questionnaire and the FI may capture different forms of frailty dimensions, being the former probably more associated with the phenotype model and the latter more associated with multimorbidity.