Background: Severe psychiatric disorders are frequently associated with disruptions in health-related behaviors, including diet and lifestyle. This cross-sectional study aimed to assess and compare selected dietary and lifestyle behaviors among long-term psychiatric inpatients diagnosed with unspecified dementia (F03) or organic delusional disorder (F06.2) and a control group of mentally healthy individuals. Methods: A 50-item validated questionnaire was administered to 28 hospitalized patients and 10 control participants. Analyses included nutritional habits, physical activity, stimulant use, and hydration, using non-parametric tests and effect size indicators (Cramér’s V). Results: Significant differences were observed in meal regularity, frequency of meals, types of beverages consumed, and physical activity. Strong associations were found for meal types (V = 0.590) and stress-induced eating (V = 0.525). Conclusions: The observed behavioral differences may reflect disease-related effects, demographic variation, or a combination of both. Despite these limitations, the findings suggest key areas for further investigation and support the need for targeted dietary and lifestyle interventions in psychiatric settings.