Obsessive-compulsive disorder (OCD) is difficult to treat, and more so when comorbid with major depressive disorder (MDD). The aim of the present case study was to examine effects of behavioral activation (BA) and pharmacotherapy with an adult with chronic comorbid OCD and MDD. BA aimed at increasing approach behaviors in life activities and decreasing avoidant and inactive behaviors. After 21 months of treatment at a community mental health clinic, OCD and MDD symptoms, including compulsive checking behaviors, were no longer at clinical levels. Symptom alleviation and psychological health improved in line with increases in activities of living such as self-care, domestic, social, and studying, and decreases in medications from a regimen of mood stabilizers and anxiolytics to a sole antidepressant. The participant was satisfied with treatment procedures and outcome. The results add to growing evidence of effective BA treatments for comorbid disorders that include depression.