Mental disorders are among the most common and most burdensome complications in oncology. They include depression, anxiety, adjustment disorders, cancer-related fatigue, cognitive deficits, and symptoms of post-traumatic stress. Their presence reduces quality of life, impairs adherence to treatment, and worsens prognosis. Among soldiers and uniformed service personnel, these conditions pose an additional challenge, as they coincide with occupational demands and specific stressors. The aim of this study was to review current evidence on the epidemiology, diagnosis, and management of mental disorders in oncology patients, with particular emphasis on the military and uniformed services context in Poland. A review of the literature published between 2019 and 2025 (PubMed, Scopus, Web of Science) and of international guidelines (ESMO, NCCN, ASCO) was conducted. Diagnostic frameworks (ICD-11, DSM-5-TR) and validated psycho-oncological screening instruments were included in the analysis. Depression affects approximately 20–25% of patients with cancer, while anxiety disorders occur in 10–30%. Cancer-related fatigue is reported by up to 40% of patients. The highest risk is observed in individuals with lung, pancreatic, and head and neck cancers. Diagnosis requires combining structured clinical interviews with screening instruments (PHQ-9, HADS, GAD-7, DT). Exercise interventions have been shown to reduce symptoms of depression and anxiety, while early clinical trials of psilocybin suggest its potential efficacy in treatment-resistant depression. In military settings, systematic screening, strict confidentiality of medical data, and interdisciplinary care are of particular importance. Mental health disorders should be considered integral components of oncological care. In the military and uniformed services, priorities include routine psychological screening at key stages of therapy, adaptation of international guidelines to national conditions, and exploration of innovative interventions (physical activity, psychedelic-assisted therapies). Such measures may significantly improve outcomes and quality of life in oncology patients.