Nutritional deficiencies are increasingly recognized as important contributors to both mood disturbance and cognitive decline. Because the brain depends on adequate vitamins, minerals, and essential fatty acids for neurotransmission, energy metabolism, and neuronal stability, poor nutritional status may weaken both emotional and cognitive health. Existing literature has linked deficiencies in vitamin D, vitamin B12, omega-3 fatty acids, iron, folate, and related micronutrients with depression, anxiety, fatigue, memory weakness, and reduced cognitive clarity. However, much of this research has examined mood and cognition separately or has focused on single nutrients, which limits an integrated understanding of their combined effects. Synthesizing evidence in a clinically relevant and interconnected way was the primary objective of this study. The article examines the primary nutritional deficiencies and their mood-related and cognitive outcomes and describes the remaining evidence. The article shows how these deficiencies may overlap and suggests multiple emotional and mental processes may be most adversely impacted by deficiencies of vitamin D, vitamin B12, and omega-3. This article stresses the importance of assessing nutrition when evaluating mental and cognitive health.