Older adults with mental illness (MI) are a highly vulnerable population and need to be provided healthcare services in a timely and thorough way. Compared with older adults without MI, older adults with MI spend a great deal of time being hospitalized and hence costing millions of dollars because the care they need is often overlooked and/or not provided. While they end up spending too much time in hospital, in the emergency department and getting readmitted to hospital because of their MI, this could have been prevented or lessened if an adequate assessment and treatment regime was done by clinicians who were well informed on the topic of older adults' mental health. Older adults with MI are also more likely to leave hospital for long-term care settings, to die and to have more sickness compared with older adults who do not have MI. Further, they are also more likely to be admitted to hospital on an urgent, unplanned basis. How older adults with MI use acute hospitals is important for psychiatric nurses to know and understand, as they can help to provide the care needed so they do not have to be in hospital for long periods of time. Psychiatric nurses can share much support and information on making sure older adults with MI are accurately care for when needed. To explore and compare predictors of hospital length of stay (LOS), acute LOS (ALOS), emergency room (ER) wait times, rate of readmission (ROR) and costs of inpatient hospital care for older adults with and without mental illness (MI) diagnoses in the province of Newfoundland and Labrador (NL). This descriptive-comparative study used aggregate population level data of 12,283 people aged 65 years and older admitted to an acute care hospital in the province of NL. A total of 8.3% of hospitalized older adults had MI diagnoses. Older adults with MI diagnoses had a significantly longer LOS, ALOS, ROR, ER wait time and costs compared with older adults without MI diagnoses, after controlling for medical co-morbidities. Key variables such as patient demographics, admission indicators, discharge indicators and other medical co-morbidities had differential impacts on observed service use. While only a small percentage of hospitalized older adults had MI diagnoses, the use and cost of acute hospitalizations was significantly greater than that of older adults without MI diagnoses.