Mary Cromwell Jarrett was a psychiatric social worker, educator, and the founding director of the Smith College School for Social Work. She was born in Baltimore, Maryland circa 1876, earned an A.B. from Goucher College in 1900, and for the next several years held teaching and tutoring positions at schools in Pennsylvania and Maryland.

From 1903 to 1913, Jarrett was employed by the Boston Children's Aid Society, and it was there that she first ventured into the field of social work, specializing in delinquent children and unmarried mothers, as well as serving as a probation officer in Juvenile Court. Jarrett went on to become Chief of Social Service at the Boston Psychopathic Hospital (1913-19), where, in collaboration with Dr. Elmer Ernest Southard, she first defined the relevance of social work to psychiatry. As an outgrowth of this work, she subsequently developed an apprentice course to train social workers in providing aid to shell-shocked soldiers returning from World War I.

In 1918 Smith College President William Allan Neilson, seeking a way for the college to contribute to the war effort, invited Jarrett to implement her program there under the auspices of the National Committee for Mental Hygiene. This was the first session of the Smith College Training School of Psychiatric Social Work, which became the Smith College School for Social Work. Its immediate success led the Board of Trustees to make it a permanent program, and in 1919 Stuart Chapin was appointed Director. Jarrett was named Associate Director, and served in that capacity until 1923.

While at Smith, Jarrett continued to conduct research and to write, notably The Mental Hygiene of Industry (1920) in which she articulated the relationship of mental health to productivity and the role of the social worker in maintaining the former. She also published The Kingdom of Evils (co-authored with E.E. Southard), a landmark book which helped establish psychiatric social work as a credible and useful adjunct to established medical practice.

In 1920, Jarrett organized the Psychiatric Social Workers' Club, which later became the Psychiatric Section of the National Association of Social Workers. From 1923 to 1925 she worked in the division of Field Investigations of Mental Health, United States Public Health Service, where she conducted a comprehensive study of the mental health of immigrants. Jarrett went on to work for the Research Bureau of the Welfare Council of New York City from 1927-43 where she directed two major studies, on health services and chronic illness. She also conducted documentation and a study of two Works Projects Administration projects: the Housekeeping Service and the Homemaker Service for the aged and the chronically ill (1935-40). After leaving the Welfare Council of New York City in 1943 and until her retirement in 1949, Jarrett conducted studies, surveys and consultations at the municipal, state and national level, specializing in old age, chronic illness and the importance of social work in helping communities and individuals to cope with these conditions. She died in New York City on August 4, 1961.

Jarrett was known for her role in developing the concept of "psychiatric social work", an outgrowth of the larger mental hygiene movement which swept the nation in the early part of the twentieth century. In Jarrett's words, as psychiatrists learned more about the nature of mental disorders, "the social problem of public mental health...increased from a matter of providing hospitals for the sick to an endeavor to promote mental development and prevent mental disease." Jarrett was initially interested in how the properly trained social worker might facilitate the work of the psychiatrist, first by obtaining a detailed history from the patient's community (as an aid to correct diagnosis) and later by helping to bring about changes in the patient's environment necessary to his or her mental well-being. However, although social workers had long been versed in assisting people with physical, mental and emotional impairments to function on a basic level, Jarrett believed that social work was destined to become "a professional art in its own right, based upon a body of sociological theory" and incorporating basic psychiatric principles.

The course of study that Jarrett developed, and implemented at Smith College, therefore was firmly grounded in sociology, psychology and social psychiatry, as well as the traditional subjects of hygiene, occupational therapy, and the writing of records and reports. Jarrett also conceived and implemented the "block plan", a novel approach to curriculum in which students alternated academic course work with internships in the field, a system still in place today at the Smith College School for Social Work.

Jarrett was certain that the same principles of psychiatric social work proven effective in treating shell-shocked veterans were also applicable to other kinds of mental distress, and she was influential in the evolution of social work into a respected profession and a viable alternative to hospitalization or institutionalization for the aged, the chronically ill, and the mentally and emotionally disturbed.