Background: We compared the role played by mental health comorbidities in the health care cost and utilization of non-elderly patients with breast and prostate cancer. Methods: The Military Health System affords its beneficiaries equal access to medical care. We performed a cross-sectional analysis using administrative data of all 9.5 million beneficiaries, ages 18-64, with direct care and a primary diagnosis of either invasive breast or prostate cancer during FY2007–FY2014. We used regression models to identify predictors of cost and utilization, including sociodemographic variables, system of care, treatment modalities, chronic disease comorbidities, and mental health comorbidities. Results: On average,annuallythere were 23,800 and 13,300 patients with breast or prostate cancer, respectively. More comorbid depression (16.8%) or anxiety (14.2%) occurred among breast cancer patients than prostate cancer patients (6.9%, 6.7%). Annual cost per patient (inc. pharmacy) was significantly higher for breast cancer ($16,287 vs. $11,069, p < 0.001). The strongest predictors of annual breast cancer costs were (in order from highest predictive value) chemotherapy, surgery, and mood or adjustment disorder (p < 0.0001); the strongest predictors for prostate costs were chemotherapy, radiation therapy, surgery, and mood or adjustment disorder (p < 0.0001). Mood and adjustment disorders were strong predictors of the annual number of ambulatory visits, hospital admissions, and bed days for both breast and prostate cancer (p < 0.0001). Conclusions: Mental health comorbidities play a significant role in health care costs and utilization of non-elderly adults who are breast and prostate cancer patients. An unexpected finding was that, although different in frequency, mental health comorbidities were important predictors of cost and utilization for both patients with breast and prostate cancer. For both breast and prostate cancer, routine screening and treatment for mental health disorders should be part of quality cancer care.