At this time, the United States had just entered World War I, and Mulliken took a position at American University in Washington, D.C., making poison gas under James B. Conant. After nine months, he was drafted into the Army's Chemical Warfare Service, but continued on the same task. His laboratory techniques left much to be desired, and he was out of service for months with burns. Later he contracted a bad case of influenza, and was still hospitalized at war's end.