Human life expectancy has increased, nationally and internationally, during recent years and will continue to increase in the future. Old age is commonly associated with health-related impairments, chronic diseases, increasing multimorbidity, as well as with psychosocial burdens, such as social deprivation, reduced social participation, and impairment in activities of daily living. Against this background, studies on mental health addressing psychological distress and mental disorders of older people have gained increasing attention. The aim of the present overview is to provide insight into psychological comorbidity in elderly with chronic illnesses. The following questions are addressed: (1) what are the prevalence rates of mental disorders in the elderly in general and, specifically, in patients with cancer? (2) How are mental disorders and cancer, respectively, diagnosed in elderly patients? (3) What are common risk factors associated with the development of mental disorders? (4) Which treatment options are available, and which aspects of health care for elderly patients with chronic diseases need to be considered?