The objectives of the study were to compare health care expenditures between adults with and without mental illness among individuals with obesity and chronic physical illness. We performed a cross-sectional analysis of 2440 adults (older than age 21) with obesity using a nationally representative survey of households, the Medical Expenditure Panel Survey. Chronic physical illness consisted of self-reported asthma, diabetes, heart disease, hypertension, or osteoarthritis. Mental illness included affective disorders; anxiety, somatoform, dissociative, personality disorders; and schizophrenia. Utilization and expenditures by type of service (total, inpatient, outpatient, emergency room, pharmacy, and other) were the dependent variables. Chi-square tests, logistic regression on likelihood of use, and ordinary least squares regression on logged expenditures among users were performed. All regressions controlled for gender, race/ethnicity, age, martial status, region, education, employment, poverty status, health insurance, smoking, and exercise. All analyses accounted for the complex design of the survey. We found that 25% of adults with obesity and physical illness had a mental illness. The average total expenditures for obese adults with physical illness and mental illness were $9897; average expenditures were $6584 for those with physical illness only. Mean pharmacy expenditures for obese adults with physical illness and mental illness and for those with physical illness only were $3343 and $1756, respectively. After controlling for all independent variables, among adults with obesity and physical illness, those with mental illness were more likely to use emergency services and had higher total, outpatient, and pharmaceutical expenditures than those without mental illness. Among individuals with obesity and chronic physical illness, expenditures increased when mental illness is added. Our study findings suggest cost-savings efforts should examine the reasons for high utilization and expenditures for those with obesity, chronic physical illness, and mental illness.