Mental health encompasses cognitive, emotional, and behavioral well-being with mental disorders defined by clinically significant disturbances in cognition, emotion regulation, or behavior. Mental health disorders are diverse and may vary within diagnoses, impacting healthcare and outcomes individually. Mental illness (MI) primarily affects cognition and emotion, including depression, anxiety, and bipolar disorder, while behavioral health disorders (BHD) primarily affect behavior, including substance abuse, eating disorders, and sleep disorders. MI and BHD prevalence is higher in cancer patients, affecting surgical and oncology outcomes. MI correlates with less chemotherapy, poorer surgical outcomes, and increased complications, readmissions, and mortality. MI also leads to increased post-discharge healthcare expenses. Suicidal ideation (SI) risk factors include mental health diagnoses, race, marital status, and gender, while intensive cancer treatment is associated with SI risk. Access to mental healthcare can be limited, particularly in shortage areas. Expanding access through incentive-based programs, collaborative care models, and telehealth systems may mitigate disparities in mental health care access and improve outcomes for cancer patients with comorbid mental illness.