U.S. social-welfare worker and early advocate of occupational therapy for the mentally ill.

While a social worker, Slagle became interested in the new field of occupational therapy, and in 1917 she conducted occupational therapy training courses at Hull House in Chicago. From 1918 to 1922, she directed occupational therapy research for the state of Illinois and then organized a therapy program for the state's mental hospitals. In 1922 she was named New York state director of occupational therapy, a post she held until her death. She demonstrated the first large-scale occupational therapy program for a state hospital system and also founded an annual training institute for state therapists that became a model for similar programs throughout the United States.

Slagle was one of the founders of the American Occupational Therapy Association in 1917.